Seto No Hanayome

A young boy gets saved from drowning by a mermaid but, according to mermaid law, if a human sees a mermaid's true form, both have to be killed. The only solution to the problem is for him to marry her. But can they keep her secret from being found out?

(Source: Animewaves)

Chapter count includes 3 special chapters.

Volume 5 contains the side story "Senjou no Merry Christmas" (瀬戸組〈せんじょう〉のメリークリスマス).
Volume 12 contains the side story "Toki wo Kakeru Shoujo" (時をかける少女).
Volume 13 contains the side story "Mission:Impossible 2" (M:I-2).