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Since ancient times, some cultures have had a superstitious fear of the ocean. It was vast and almost completely unknown. It could smash any ship man dared float on it, or sweep it away, never to be heard of again. Oh, and there may have been gigantic monsters in it. Until recent centuries many sea charts were illustrated with mythical and fearsome sea-creatures in unexplored regions, and even today the weirdest creatures on Earth are to be found beneath the waves, and we're still finding more. Some of them are pretty enormous, too (though they're not the most dangerous things down there).
There are many flavours of Sea Monster (well, yes, most taste a lot like squid, unsurprisingly). These subtropes include:
- Giant Enemy Crab
Really big crabs. Well, other crustaceans, too. They all taste like a lot like crab, though.
- Giant Squid
Really big squid. Older Than Feudalism, they have the benefit of actually existing. Makes great calamari.
- Kraken and Leviathan
The epically scaled, humongous world-ending dwellers of the deepest trenches, often with lots of references to Lovecraftian or Biblical beasts. Tastes like whale meat.
This prehistoric beast is great for those who like to buy their Shark Fin soup in economy scale.
- Stock Ness Monster
Those that ape (or more like serpent) the queen of cryptozoology, her royal slitherness the Loch Ness Monster. Said to taste rather exotic, like crocodile meat.
- Turtle Island
Those whose large size combined with a sedentary life in shallower conditions make them liable to be confused for islands. Apparently the taste of giant turtle is meant to be greater than any chicken, beef, mutton or butter and a favourite for long sea voyages.
Please place examples belonging to those subtropes only in the specific page, not here.
Anime & Manga
- The Three-Tails and Eight-Tails from Naruto.
- There's also a giant squid that was the guardian of an island where Killer B and A trained.
- One of the magical creatures in Magical Girl Lyrical Nanoha from which the Wolkenritter took Mana was a many-eyed giant sea monster that emerged from a whirlpool.
- One Piece, being a pirate-themed adventure, has lots of sea monsters — the sea kings being a pretty prevalent example.
- Digimon has an entire group of digimon devoted to this. Of particular note are the Seadramon family, which includes the season one baddie Metal Seadramon, and the enormous Demon Lord, Leviamon.
- Water Pokemon. Let us see, we have giant crabs (Kingler), giant lobsters (Crawdaunt), giant jellyfish (Tentacruel), a mini Loch-Ness monster thing (Lapras), giant clams (Clamperl and Cloyster), a torpedo shark (Sharpedo. Yeah.), ancient water animal plant things (Lileep, Anorith)...and the waters hide many, many more.
- A sea serpent appears in The Sandman story "Hob's Leviathan", part of the "World's End" arc.
- The oceans of Skataris are filled with monsters in The Warlord.
- So are the oceans and rivers of Elekton in The Trigan Empire
- Giganto/Monstro, the Sub-Mariner's giant whale with arms and legs in Fantastic Four.
- A series of Jameson Irish whiskey commercials feature John Jameson jumping overboard to retrieve a barrel of booze. A giant octopus is seen reaching towards him underwater in some versions. Viewable here.
- The Beast from 20,000 Fathoms.
- Jaws and its host of imitators.
- The bluntly titled Godzilla versus the Sea Monster.
- Heck, Godzilla himself certainly applies. In the first film, he's even shown sleeping underwater.
- Likewise, there's Manda, who is a sea-dwelling Chinese Dragon-looking guardian of the lost civilization of Mu.
- And several others, like Gezora, Titanosaurus, Zilla, Ganimes, and Maguma.
- There's Always a Bigger Fish on Naboo. First, there's the Opee Seakiller, a bizarre fish-crab hybrid that eats with a long, sticky tongue. Then, there's the larger Colo Clawfish, a giant sea serpent with clawlike appendages used to capture smaller creatues (especially baby Seakillers, which can even chew out of a Clawfish's stomach to escape). And finally, there's the Sando aqua monster, the biggest predator in Naboo, who can and will eat almost anything (including the aforementioned Clawfish and Seakiller) that stands in its way.
- The Avanc (from the afanc of Welsh mythology) from China Mieville's The Scar.
- Lord of the Flies also has the whole "beast from sea" speculation affair.
- The Watcher in the Water from Lord of the Rings.
- The sea serpent from Voyage of the Dawn Treader.
- The creature in Ray Bradbury's short story The Fog Horn, which was the inspiration behind The Beast From 20,000 Fathoms.
- In Orlando Furioso, two different women, Angelica and Olympia, were offered to placate two different sea monsters — at least one of which was called an orc. Also, the enchantress Alcina uses a whale as transport.
- Discworld - The illustrated novel The Last Hero features a map of the disk with all sorts of monstrous sea creatures popping up here and there. Then a later illustration shows Mustrum Ridcully fishing... with all those creatures lying in a pile at his feet. Turns out the creatures on the map weren't quite on scale.
- The sentient sea serpents in Robin Hobb's Liveship Traders trilogy.
- In the massive live-action RPG tournament from The California Voodoo Game, the aliens responsible for all the weird events were stranded on Earth because they'd grown into adulthood since their ship crashed. Their mature forms were those of massive flatfishes more than 50' long, so they could no longer fit into their spaceship. Too bad the players never made it back up to the roof, to sneak a peek at the swimming pool....
- Sense and Sensibility and Sea Monsters lives on this trope.
- Taylor Anderson's Destroyermen takes this trope to an insane level. In this alternate world, the great white shark is a small predator with loch ness style creatures being average size and everything being dwarfed by the monster fish, a sea creature so ridiculously enormous that it could swallow an aircraft carrier in one bite.
- Percy Jackson encounters the legendary Charybdis and Scylla in The Sea of Monsters, a random sea serpent in the short story Percy Jackson and the Stolen Chariot from The Demigods Files, and sees a whole army of them attack Poisden's forces in The Last Olympian.
Live Action TV
- Primeval has featured a few prehistoic sea monsters. Episode 1.3 gave us a Mosasaur, Episode 2.4 gave us a future shark and the Mer, walrus-like sea monsters that appeared to have evolved from primates, and Episode 5.2 gave us a whole pack of Pliosaurs.
Myth And Legend
- Older Than Feudalism: Charybdis, from The Odyssey, an unseen monster that swallows and spits out the entire ocean on a regular basis in the form of a giant whirlpool. Related is Scylla, who lives near the ocean rather than in it and has lots of heads 
- Another even older Older Than Dirt example: Tiamat, the primordial sea serpent from the Babylonian creation myth Enuma Elish.
- Many a woman Chained to a Rock was to appease a Sea Monster. From Greek legends, Hesione and Andromeda's monsters. Andromeda's was named "Cetus," or "Ceto," which of course means "whale" now.
- To add insult to Ceto's mortal injury, after Perseus slashed her throat, he delivered the deathblow by showing her the petrifying head of her own daughter, Medusa, transforming Ceto into a coral reef.
- The Con Rit, or Tarrasque, a sea serpent claimed to be found in the South China Sea. Some theories say that the possible sea serpent is inspiration to both the Oriental Dragon, and the South African Snake God. 
- The Kalevala has a giant pike and Iku-Turso, a malevolent sea monster.
- Norse Mythology has the Kraken of course. And then there's Jormungandr, the Midgard (World) Serpent, the sea monster to end all sea monsters. An immense sea snake, he wraps all the way around Midgard, holding his tail in his own jaws. He has a major rivalry with Thor, and plays a key role in Ragnarok.
- Magic: The Gathering has tons of Sea Monsters. Leviathans, Krakens, Sea Serpents etc... Notably enough, they're often both blue and absurdly huge, despite blue colour being anything but based on brute force.
- Rifts has boatloads of sea, lake, and even river monsters.
- Dungeons and Dragons
- Sea Monsters by the truckload: scyllans, krakens, leviathans, ramfish, dragon turtles, sea drakes.
- And then there's Dagon, a lesser-known demon prince who is an eel with the head of a freaky fish, six tentacles ending in clawed hands, and more eyes than I really want to count. Other demon princes are afraid of him.
- The ocean-oriented Stormwrack book includes many Sea Monsters.
- Older editions had the Behemoth (giant hippo), aquatic "dinosaurs" (Elasmosaurus, Mosasaurus, Nothosaurus, Plesiosaur), dragons (Lung Wang and Yu Lung), Froghemoth, Giant (Crayfish, Crocodile, Eel, Octopus, Sea Snake, Squid, Turtle), giant fish (Afanc, Gar, Pike, Verme), Merrow (aquatic ogre), Mottled Worm (aquatic Purple Worm), Sea Serpent, Seawolf (lycanthrope), and Vodyanoi (aquatic Umber Hulk).
- Warhammer 40000 has a few, though they're rather obscure. The Space Wolf homeworld has a massive kraken (said to be a Tyranid offshoot) and sea serpents straight out of Norse myth, appropriate given the Space Wolves' Viking theme, and a sea monster is said to live on the planet Armageddon, where it attacked Ork ships. Given the nature of the setting, it's a safe bet that most world with any oceans have at least one.
The "Iron Snakes" chapter has a mostly ocean-bound home world (their fortress-monastery is on one of the moons), and one of the challenges which aspirants wishing to join the chapter have to face is single combat with a sea serpent using only a harpoon and a canoe.
- Several in Bionicle, particularly the Mahri Nui arc, including Gadunka( basically a giant, toothy mouth with small limbs attached), a 300-foot long eel-thing, and cyborg sharks.
- The Water Dragon from Okami, which turns out to be the king of the Dragonians.
- The Sapphire Drake Bonus Boss of Wild Arms 4, which the player first sees preying on another sea monster.
- Kyogre fills the 'primal sea monster' niche, while lesser creatures such as Gyarados are based on sea serpents.
- Milotic is considered the benevolent counterpart to Gyarados due to the fact that it's also a serpent, opposite temperament and the fact it evolves from an equally weak Pokemon though it is much rarer.
- Gold/Silver/Crystal features a Red Gyarados (most are blue) rampaging in a lake. In the game a Red Gyarardos evolves from a Golden Magikarp
- The Sega arcade shooter The Ocean Hunter has its share of giant sea creatures, the biggest of which serve as boss and mini-boss fights.
- La-Mulana is maybe the only video game to use the Babylonian interpretation of Tiamat (fierce ocean goddess) rather than the Dungeons and Dragons interpretation (five-headed dragon queen).
- Team Fortress 2 - The Demoman considers the Loch Ness Monster to be his mortal enemy, and he accidentally killed his adoptive parents in an attempt to blow it up.
- Del Lago from Resident Evil 4 resembles some kinda giant newt.
- Banjo-Tooie - The boss of the obligatory underwater level is Lord Woo Fak Fak, a gigantic lanternfish who literally lives in Davey Jones' Locker. He can be a bit tricky, but is much easier to fight if you use the Submarine transformation.
Also in Jolly Roger's Lagoon is a giant fish that swallowed Merry Maggie the barmaid. Strangely, it doesn't try to attack you, even when you knock all the teeth out of its mouth in order to rescue her.
- There are some large aquatic creatures in World of Warcraft. Notable examples include Ghaz'rankha, a giant hydra that's a boss in Zul'gurub raid instance (gaint hydras also appear as bosses in Zul'zarrak and the Underbog), and The Lurker Below, a boss in Serpentshrine Cavern raid whose ingame model is actually called "kraken". Some of the larger naga may count for this trope too, their queen Azhara most certainly does.
Hydra in World of Warcraft are lizards with three heads. They have two legs and a heavy tail, so they basically move on a tripod as well. Normal ones are about the size of a wooly mammoth or so, but bosses are a little bigger. The Lurker Below is big enough to be a genuine kraken, and three other things in the game share its same ingame model: two similar wild animals related to specific quests and a sea goddess.
- Super Mario
- Super Mario 64 introduced Unagi the giant eel  and the Sea Dragon — who actually lived in an underground lake. The Sea Monster's name is "Dorrie", although in Japanese it's name is "Doshi", which is a parody of the Loch Ness Monster and Yoshi since the nickname "Nessie" in Japanese is "Neshi".
- Super Mario Sunshine had a boss named Eely-Mouth, who was more goofy-looking.
- Niptra of Arcana Heart, an ancient fish that became the largest and wisest fish in the seas through cycles of Reincarnation before finally becoming the Arcana of Water.
- The Shiguras from Dark Cloud 2, extremely reminiscent of Pokémon's Lapras. Dr. Jaming intends to use them as tools for his own nefarious purposes, and goes as far as to enslave one of them, Pau's friend, Shingala.
- Aquanaut's Holiday has several whales swimming in its oceans, but there's also an enormous prehistoric fish floating in the southwest corner of the map.
- Monster Rancher had the Zilla, a freaky whale/bear hybrid.
- Morpheel in The Legend of Zelda: Twilight Princess
- One of the later puzzles in Azada: Ancient Magic requires you to free the Baron (Munchausen) from a giant sea monster's mouth.
- Monster Hunter Tri features new mechanics which allow your character to swim and fight underwater. Obviously this has introduced a whole slew of new aquatic monsters including a lightning-spewing serpent/crocodile mix and a large, grotesque Angler Fish capable of perfect camoflage, blinding flashes of light and inflating into a spined ball.
- A boss in Metroid Fusion is Serris, a sea serpent capable of Super Speed.
- Shadow of the Colossus features two: the electric eel colossus that is frankly terrifying, and the wading lake monster, which is terrifying AND frustrating.
- Endless Ocean: Blue World features several prehistory-dating creatures to be encountered, one of whom even tries to attack you. There's also lots of benign marine life which have monster-esque names, such as a large giant squid named "Kraken Jr." and an albino sperm whale named "Leviathan". Sadly, all of the prehistoric creatures are relegated to cutscenes only and cannot be swam with.
- Actually, Kraken Jr. is the giant squid's child - he's actually quite small, but he may still count since he'll eventually grow into a giant.
- Expansion packs for the original Zoo Tycoon allow you to raise and exhibit plesiosaurs and Loch Ness monsters. Not together, though: they eat each other's eggs!
- Tradewinds: Legends and Odyssey have several sea-monster battles, both bosses and random encounters.
- Also, in the Legends Captain's Log: "Fought kraken. Looking for good calamari recipe."
- Nintendo game Cobra Triangle featured several different sea monsters to include a Giant Enemy Crab, a Giant Squid, sea serpents and a giant shark as the final boss.
- This is the entire point of the World Monster Fishing mode of Big Ol' Bass 2, an obscure but awesome fishing game. You can catch everything from bass with the American flag colors to a trout with the Mona Lisa on it, from a shark with a leopard design to Dinosaurs.
- Oblivion has the Giant Slaughterfish.
- My Little Pony Friendship Is Magic features a metrosexual sea river serpent whose hysterics over having his good looks ruined in a medium-term sort of way cause our heroes a problem in the second episode.
- Avatar: The Last Airbender
- There is a reason that the Serpent's Pass in was called such, which the characters find out first hand when the giant sea serpent came out when they tried to cross.
- The giant lion-turtle with an island on its back.
- And the Unagi that lives near Kyoshi Island.
- During the Season 1 finale, the ocean spirit assumes the form of a huge, humanoid, Godzilla-esque monstrosity.
- Cecil the Sea-Sick Sea Serpent from Beany and Cecil
- Bad guy Dishonest John had an undersea craft disguised as a giant cephalopod, known and feared far and wide as Billy the Squid.
- Catscratch features a Kraken, which Gordon must fight in order to wish for a longer tail.
- The pilot of The Drinky Crow Show involves whales, which is appropriate because it's set on a whaling ship. The whales are drawn in the style of those really old maps with "Here be dragons" written in the corners.
- Star Wars: The Clone Wars - Jar-Jar summons a bigger fish.
- SpongeBob SquarePants - When bob yells that there are sea monsters on the beach to keep people out of the water, an erudite serpent reminds him that "we sea monsters have made great strides in the fields of science and literature."
- The giant Nautilus of Cybersix. She beats it by dropping a bridge on it, literally.
- Care Bears had a sea serpent named "Shaky" who needed work on his confidence.
- In other Star Wars: Clone Wars the Mon Calamari fought the separatist forces with knights riding on giant eels.
- DuckTales: The sea monster that ate Scrooge McDuck's ice cream is actually a submarine colored to resemble an orca.
- Yellow Submarine had an entire Sea of Monsters.
- Whales, obviously.
- And including extinct animals, we have plesiosaurs and similar creatures like the ichthyosaur and mosasaur, and Megalodon, a prehistoric shark the size of a whale. Its modern counterpart may be the Whale Shark, which is thankfully nonaggressive and feeds on plankton.
Indeed, so many different creatures like this existed in the past that one can argue that plesiosaurs can't have survived to the present, because the mosasaurs would've eaten them, long before the K-T extinction took down subsequent forms of reptilian Sea Monster.
The modern equivalent of Carcharocles megalodon might be the Whale Shark from a size perspective, but in every other respect it's basically a Great White that is... bigger.
- Check out the miniseries Sea Monsters (from the creators of Walking with Dinosaurs) for some of the nastiest aquatic predators ever.
- Predator X. Its teeth are each a foot long.
- The Oarfish: an eel-like fish that can exceed fifty feet long. Harmless, but quite impressive.
- The African country of Burundi is home to Gustave, a well-known man eating crocodile. Gustave weighs in at approximately one ton, is approximately twenty feet in length, and has been known to eat adult hippopotami. Given the number and variety of bullet-shaped scars which cover his body, it's quite possible that he's Immune to Bullets.
- Saltwater crocodiles can be even larger than Gustave. Large adult male saltwater crocodiles can be more than six meters long and weigh more than 2,600 lbs.
- The Bloop, or rather, whatever made the Bloop, an ultra-low frequency underwater sound that matched the profile of a living creature, but not any specific living creature we know. If it did come from a living creature, scientists agree that it would have to be several times larger than the Blue Whale, the largest known animal on earth.
- Tullimonstrum, a bizarre fossil invertebrate, bears a striking external resemblence to the Loch Ness Monster, except it's barely a foot long and found in North America.
- The Coelacanth. Now, fair enough — as fish go, they're small and rather harmless. But the fact the coelocanth has been swimming around virtually unchanged for millions and millions of years, makes a lot of supposed sea-monster sightings just a little bit creepier.
- A person-sized carnivorous fish is not harmless.
- The whole point of fishing shows like River Monsters, Hooked, and Monster Fish. Their catches range from the modest (Taimen), to the bizzare (catfishes of the Amazon), to the truly gargantuan in scale (marlins, giant catfishes, and the Mekong Giant Stingray).
- The basking shark, second-largest living fish, can grow up to thirty-plus feet. They also look rather monstrous, cruising through the water with their three-foot mouths. Fortunately for all mankind, they and whale sharks eat only plankton. They are also the source of a few sea monster myths, where the decomposing bodies of these sharks after having beached or caught as by-catch have been mistaken for plesiosaurs.
- Several deep-sea species of shark are also quite large. The megamouth shark grows to eighteen feet in length, and the Pacific sleeper shark can grow to fourteen, though ichthyologists theorize they may grow as long as twenty-three feet. Related to the Pacific sleeper is the Greenland shark. It lives in Greenland and closer to the surface, routinely grows to seventeen feet in length and has been found with parts of polar bears in their stomachs.
- Lion's mane and Nomura's jellyfish can grow to gigantic sizes. The lion's mane can grow nearly forty feet long and the Nomura can weigh over four hundred pounds. Nomura's jellyfish have also marked off an important check on the "to-do list" of most sea monsters; sinking ships, albeit in a passive manner. "Blooms" of these jellyfish have numbered in the thousands and at least one fishing vessel has been capsized after trying to haul up a net full of these jellyfish.
- The largest lion's mane to wash up had tentacles in excess of 120 feet. That's longer than a blue whale, usually considered the largest organism. Now imagine encountering that while deep-sea diving...
- Syringammina fragilissima. It's a deep-water denizen eight inches across. It's not very impressive-sounding, but suddenly becomes more so when you find out it's a single-celled organism.
- Scylla used to be a sea nymph. She was courted by the god Glaucus, but spurned his advances. Glaucus went to the sorceress Circe for a love potion. Circe, falling for Glaucus, turned her "rival" Scylla into a monster in a fit of angry jealousy. Greek Myth: a soap opera, but hardcore.
- In French mythology the Tarrasque was originally a tortoise-shelled, eight legged land bound dragon that menaced the French region of Tarrascon (thus the name) until defeated by a female saint. The French drew on this legend for their name for the beast of Halong Bay while Vietnam was still part of French Indochine.
- who has since made appearances in New Super Mario Brothers and Mario Kart Wii.