Umi Monogatari ~Anata ga Ite Kureta Koto~(2009)

Umi Monogatari ~Anata ga Ite Kureta Koto~

The series’ story centers on Marin and Urin, two mermaid sisters that long to visit terra firma. When the sisters find a ring that has fallen into the sea, they decide to venture onto land to return the ring to its high school girl owner.

(Source: Manga-Updates)