Orenchi no Furo Jijou (The Circumstances in My Home's Bathtub)
The story begins with Tatsumi, a high school boy who lives by himself. Then, a devastatingly handsome merman named Wakasa moves into the bathtub in Tatsumi's home. Wakasa is a bit self-centered but cute, while Tatsumi is cool but a busybody. The story offers a peek into their lives together.
[Source: manga synopsis via Anime News Network]