The 'aquatic ape' hypothesis (it can't be described as a theory) has been around for quite a while, & in fact I've blogged about it before. So I was sorry to hear that Sir David Attenborough, who's done so much to promote conservation issues and enhance our understanding of the natural world, appeared to have given the idea some support. He's certainly taken some flak for this (see here, for example), although at the same time other - ahem! - news outlets have picked up the ball and trotted off down the garden path with it.
Briefly, the aquatic ape hypothesis (I will NOT call it a theory) purports to explain the evolution of a number of aspects of our morphology: our relative hairlessness & the distribution of that hair, bipedalism, the way so many people like fish (I will put my hand up as an exception to this), distribution of body fat, & so on. ** Unfortunately for this particular just-so story, there's good evidence that all these features did not evolve at the same time. Bipedalism, for example, pre-dates the chimp-human divergence, but the addition of fish to the diet seems to have appeared much later. Nor is there necessarily strong evidence of any links between a particular feature & the life aquatic. For example, while cetaceans are essentially hairless, seals, sealions and their relatives are covered with dense coats of fur.
Anyway, the hypothesis has recently been the focus of some entertaining parodies, among them the 'space ape' version (face-to-face copulation would really have been the only option, dontcha know? for otherwise the jetpacks would get in the way) and - as a conclusion to his explanation of why the aquatic ape idea doesn't stack up - Henry Gee's thought experiment involving the unlikely combination of elephants and custard.
** "& so on" includes the sinuses in our skulls (another feature that reinforces our African origins). Apparently they provided a buoyancy aid - yet they're found in all mammals regardless of habitat.
[EDIT] However, courtesy of one Smut Clyde I find that the aquatic ape proposal has nothing on this.