Orenchi no Furo Jijou

Synopsis

When an aloof but kindly teenage boy named Tatsumi helps out someone who'd collapsed on the banks of a river close to his home, little does he realise that he'll soon be sharing the bathroom of his modest town house with Wakasa, a self-obsessed and playful merman. Impromptu visits from Wakasa's friends - all of whom are aquatic animals with human traits - while fun, don't exactly make things easier. Welcome to Tatsumi's bathtub. It's getting a bit crowded.

Synopsis & details from MangaUpdates