ORENCHI NO FURO JIJO

Tatsumi is a high school boy who lives by himself. One day, 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. (Source: Anime News Network) — Description from AniList community (anilist.co).