Umi Monogatari.

Marin and Urin are sea folk, people who live in a peaceful underwater world surrounded by bright fish and colorful coral. But one day, a beautiful ring falls from the "sky" - that dangerous place above the water - and the two girls decide to return the lovely trinket to its owner. While the surface world holds amazing wonders for the pair, it also hides a great darkness... a darkness that Urin accidentally unleashes when she opens a stone box at a local shrine. Now it's up to Marin, Urin, and their new friend Kanon to set things right again before that darkness swallows the entire world!