Is Kono Danshi, Ningyo Hiroimashita. hentai?
English title: This Boy Caught A Merman
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

OVA · 2012 · 1 episodes · PG-13 - Teens 13 or older
Boys Love, Drama, Fantasy
Shima Kawauchi was taken in by his kind grandfather following the divorce of his unloving parents. His grandfather passes away in due time, leaving Shima all alone. Consumed by grief, he cries by the oceanside at night when an accident causes him to fall into the water. Unable to swim, Shima nearly drowns but is saved by an incredibly handsome merman. Seeing Shima's emotional state, the merman—who Shima names Isaki—insists that Shima take him home until he fully recovers. However, as they settle into their space together, the two start to form a bond that transcends both species and gender. [Written by MAL Rewrite]
View on MyAnimeList