Ningyo Shinjuu

Mizushima, a yakuza, was fascinated by a Maitreya Buddha statue as a boy and developed a nasty habit of lusting only for non-human things. One night, Mizushima encounters a mermaid in the dark sea. Mizushima cannot forget the bewitchingly beautiful mermaid... He becomes entangled with the innocent "monster" and falls into a bottomless swamp of love...

(Source: Shinkosha, translated)