Yukemuri Mermaid
湯けむり人魚姫〈マーメイド〉
A 16-year-old boy named Eikichi works at a family-run thermal bath. One day, he is rescued from drowning in the sea by an attractive girl who vows to stay and teach him how to swim. Unaware of her true identity, he accepts her help. The girl, however, is actually a mermaid princess.