My Bride is a Mermaid (Seto no Hanayome)
Nagasumi Michishio is visiting his grandmother for summer vacation when one day he is saved from drowning by a mermaid named Sun Seto. According to Mermaid law, if a human sees a mermaid's true form, both must be killed. The only solution to the problem is for him to marry her -- but can they keep her secret?