This Boy Caught a Merman

Kono Danshi, Ningyo Hiroimashita.

*The Second OVA of Kono Dan series. They say look before you leap and make sure you can swim before you go in the deep water, but when a picture of his late grandfather falls into the ocean, Shima jumps in after it without thinking. Nearly drowning as a result, he is instead saved by a very perfect stranger... one whose strangeness extends to only being human from the waist up!...