Seto no Hanayome Temporada 1 Episódio 1

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 Suns mermaid clan is in fact a Yakuza organization.