Umi Monogatari - S01

Marin and her younger sister Urin, both seafolk, stumble upon a strange sight beneath the waves-a beautiful silver ring. Determined to return it to its rightful owner, the kind-hearted Marin convinces a reluctant Urin to join her on a journey to the sky world, despite warnings about a turtle elder who once ventured there and never came back. They find the ring's owner, Kanon Miyamori, who admits she threw it into the sea after her boyfriend broke up with her that same day. Marin argues that something so precious shouldn’t be discarded, but Kanon tosses it away again. While searching for the ring, Urin gets separated and accidentally breaks the seal on a stone coffin, unleashing an ancient evil named Sedna. Sensing the danger, the long-lost turtle elder, Matsumoto, appears before Kanon and the sisters, revealing that Marin is the Priestess of the Sea-and that she, along with the Priestess of the Sky, can reseal Sedna. By chance, during a fight with one of Sedna’s minions, Kanon learns she is the Priestess of the Sky. Though unsure at first, she and Marin agree to join forces and stop Sedna before the world is destroyed.