Umi Monogatari

Marin, a seafolk, and her sister Urin find a lost silver ring beneath the waves. Marin intends to return it to its owner, Kanon Miyamori, who had previously discarded the ring after a breakup. However, Marin's efforts to locate Kanon and return the ring are complicated by her sister's disconnection from the group and the accidental release of Sedna, an evil being, from a stone coffin.