I forget which show, maybe Monster Quest (which I think originally aired on the History Channel, now available on Netflix). They examined the mermaid thing and came up with a prototype for what a human/fish hybrid would look like in order to be equipped for the underwater environment.
They came up with something that was bald, with greyish skin, loads of blubber and looked like it should be an "America's Biggest Loser" contestant. Ariel it was not.