Seto no Hanayome

Michishio Nagasumi's life couldn't be any more normal. In an odd twist of events, during his summer vacation he ends up almost drowning in the sea. Luckily, the cute mermaid Seto Sun appears to save him. However, Sun is from a yakuza mermaid family and according to their law, if a human is to catch sight of a mermaid, either he or the mermaid must die. The only other way is for Nagasumi to be taken in as a family member, marrying Sun. In attempts to save both of their lives, Nagasumi asks for Sun's hand in marriage. Nagasumi's summer vacation reaped more than what he would expect, as he must now protect Sun from others finding her secret out.