Umi Monogatari: Anata ga Ite Kureta Koto

Marin and her younger sister Urin are seafolk who happen upon something quite strange: a beautiful silver ring lost beneath the waves. The kind-hearted Marin, intent on returning it to its owner, drags a reluctant Urin along with her to the sky world despite reminders of a turtle elder who left for the surface and never returned. After locating the ring's owner, Kanon Miyamori, they learn that Kanon had tossed it into the sea after her boyfriend dumped her earlier that day. Though Marin insists that such a lovely item should not be thrown away, Kanon discards it once again. As they search for the ring, Urin becomes separated from the other two and accidently breaks the seal on a stone coffin, releasing an evil being known as Sedna. Sensing Sedna's release, the formerly missing turtle elder, Matsumoto, reveals himself to Kanon and her companions, naming Marin as the Priestess of the Sea. Together with the Priestess of the Sky, she has the power to seal Sedna away again. And as luck would have it, during an encounter with one of Sedna's minions, Kanon discovers that she is the Priestess of the Sky. Though Kanon is hesitant, she and Marin decide to work together to save the world from the evil that threatens it. Marin and her sister Urin live in the sea. One day they find a ring fallen into the sea. They get out of the sea and meet a girl Kanon, the owner of the ring. Kanon tells them she doesn't want the ring anymore because it was given by her boyfriend whom she just broke up with, and she throws it into the forest. Marin thinks Kanon is still in love with him and looks for the ring with her sister. But Urin accidentally breaks a seal near a shrine...