Shiofuki Mermaid: Poolside no Hatsutaiken.

Makoto is an ordinary university student who often visits a nearby swimming pool to swim and watch women in swimsuits. One day, he is asked to teach Marina how to swim. It seems she can't swim at all. While he touches her body, he gets an erection. Marina notices it, and gives him a blowjob. When he touches her vagina, she cries out in pleasure and achieves female ejaculation. His unknown erotic ability comes into bloom!