This Boy Caught A Merman Kono Danshi, Ningyo Hiroimashita.

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]