Seto no Hanayome [AniDB]
Terra e... [AniDB]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.
In the far future, humanity has left behind an environmentally destroyed Terra and began colonization in order to reproduce their home. Humanity, now ruled by a super computer that controls the birth of children, sees the emergence of a new race called the "Mu". The Mu, now hidden from the rest of humanity, have one dream, to return home, to Terra...