The Aquatic Ape Theory was popularised by Elaine Morgan in the face of much antagonism from mainstream scientists. You can read more here. And see her talk at TED here. Essentially she argues that at some point in our evolution we adapted to being semi-aquatic; and describes several features common to humans, semi-aquatic mammals and aquatic mammals, but not primarily land based mammals.
The comments after the You Tube clip give you a real taste of the antagonism this hypothesis arouses!
And a more recent reference to the theory in The Guardian, 27 April 2013.