My Bride is a Mermaid Season 1 Episode 1

A Yakuza's Wife

Nagasumi, a teenage boy, is saved by the chivalrous mermaid Sun while drowning in the ocean off the coast of Seto Island. Mermaid society rules are strict — in order for both Nagasumi and Sun to stay alive, they have to get married. Little does Nagasumi know that Sun's mermaid clan is in fact a Yakuza organization.