Umi Monogatari: Anata ga Ite Kureta Koto
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The story centers on Marin and Urin, two “pure” sisters who live alongside the fish in the sea, but yearn to be in the world beyond the water and above ground. One day, a beautiful ring falls in the middle of the sea, and Marin and Urin retrieve it. The two decide to leave their waterbound world for the first time to deliver the ring. After an arduous journey, they come across an isolated island where a high school girl named Kanon lives. The encounter between Marin and Kanon — the maiden of the sea and the maiden of the skies — awakens a hidden power as the world is threatened by an enveloping darkness.
-- from Wikipedia