Umi Monogatari: Anata ga Ite Kureta Koto
うみものがたり 〜あなたがいてくれたコト〜
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)