My Bride is a Mermaid

The anime series My Bride is a Mermaid is a comedy-romance story about regular teenage boy Michishio Nagasumi who gets saved from drowning by a mermaid named Seto Sun. What could be a mostly-straightforward first meeting is not so, because there's a rule that when a (shapeshifting) merperson reveals their identity to a human, one of the pair have to be killed, unless the pair are married. To make matters worse, Sun is the daughter of a yakuza-type merfolk clan leader, who is very protective of his daughter, and not happy at all that she has to marry a useless human like Nagasumi.

So begins a story of love, friendship and shenanigans as Nagasumi gets drawn into this complicated world he did not know existed, and he and his new bride Seto figure out what their relationship can be despite the less-than-ideal beginnings.