Warawanai Ningyo Manga不笑的人魚, , 笑わない人魚, , The Mermaid Who Doesn't Laugh , The Unsmiling Mermaid
From Storm in Heaven:
A collection of short stories. The title story is about Youzuke; one summer when he was growing up, he stayed at his uncle's house. His uncle had a pool, and at night Youzuke would see a mermaid swimming there, visiting his uncle. Years and years have passed, and Youzuke's uncle has disappeared. Surprisingly enough, his will leaves the house to a stranger, Ayumi Kenjou. But when Youzuke meets Ayumi, it seems like he'd already met him that summer so very long ago.