Umi Monogatari: Anata ga Ite Kureta Koto

Marin and her sister Urin find a ring beneath the waves and leave their underwater home to return it. Their search leads to Kanon, a girl who threw it away after heartbreak and has little of Marin's unguarded faith in other people.

The encounter draws them into a struggle against Sedna, a force threatening to spread darkness. As Marin and Kanon awaken as the priestesses of sea and sky, protecting their worlds depends on a bond strong enough to bridge their differences and private hurts.