Actually I find the Mermaid idea totally intriguing! But I mean the organs problem is sort of easily solvable - there are loads of stories of Meremaids (and other sea fairytail creatures) who when caught turn into a sort of humans as long as they are not in the water. In these old fables poor young lads fall in love with their beauty, steal their "cloths" (well actually their skin or something) when they are out in the air, and so make them dependant on them. But in the stories it would never last and one day the mermaid finds her skin somewhere and leaves the human, though she may have loved him and he is left alone and miserable.

I kinda would like a sexy twist to that whole theme...