From Anime New Network:
Zushio was to be king and promised his sister that he would become king (on penalty of death). Unfortunately for him, his empire was overthrown and now he's a wanted man. A girl has the misfortune of running into Prince Zushio and becomes tangled in his plight. Adding to her misfortune, Zushio is not quite right in the head, but hey, at least he seems to be immortal.

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).