Elfie of the Blue Seas

The legend of the Coral Island: Elfie of the Blue Seas (サンゴ礁伝説 青い海のエルフィ, Sango-sho Densetsu: Aoi Umi no Elfie) is a 1986 Japanese anime fantasy adventure television film directed by Yoshio Kuroda, with Sumi Shimamoto voicing Elfie herself. It was originally broadcast on May 19, 1986 on Fuji Television.

In the future, the oceans have risen to flood all the continents due to humanity's negligence of the environment. Only a few bits of land have been spared, and one of those is the city-island where a quiet young girl named Elfie lives with her grandfather.

One day, due to an incident where Elfie loses her breathing device while still submerged in the sea, she discovers that she can breathe when underwater, and her originally brown hair turns blue when this happens. Her grandfather reveals that in truth she is one of the mythical sea-people, who can survive under the sea. . .

Using the city folk's xenophobia, local politicians spark a war with these hidden people to distract people from their current resource problems. As a result of being a sea-girl raised by humans, Elfie is caught in the middle of it and she takes two decisions: to find out more about her origins and to stop the the war no matter what.

Tropes used in Elfie of the Blue Seas include:

• Adjective Noun Fred: Sort-of.
• Apparently Human Merfolk: Aside of the capacity to breath underwater and the color-changing hair, Elfie looks more or less human. So do the rest of the sea people.
• Badass Pacifist: Elfie must become this, if she wants to unite people.
• Bittersweet Ending: The war between humans and the remaining sea people doesn't happen, and they reach an agreement to live peacefully together. . . but Elfie has to give her life for that.
• Break the Cutie: The plot seems to bend over for this to happen to Elfie. She dies, yes, but refuses to yield even in almost death.
• Die or Fly: If not for being a member of the sea people, Elfie would've fatally drowned. 
• Friend to All Living Things: Elfie, especially to dolphins.
• The Great Flood: This happened in the past, and humanity can barely survive in the few dry lands left.
• Hidden Elf Village: The Underwater City that Elfie finds at the bottom of the sea.
• Last of His Kind: Elfie. Among humans, that is: the sea people are still alive underwater.
• Miss Fanservice: Downplayed, but still there regarding Elfie. It's mostly due to her clothes: her blouse has semi-Detached Sleeves that leave her shoulders bare PLUS uncovers her belly, while her flowing skirt is quite short and lets people see her cute legs.
• Orphan's Plot Trinket: Elfie's pendant.
• Power Dyes Your Hair: Elfie is a brunette normally, but her hair turns blue when she breathes underwater.
• Plucky Girl - Silk Hiding Steel: Elfie is a quiet young lady, but demonstrates great courage when things go south.
• Underwater City: Averted, humans don't have these. The surviving sea-people do, however.