O czym to anidb mal

Podwodne mahou shoujo.

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.Source: ANN

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...