The Aquatic Ape Theory in it's simplest form is that we evolved from an aquatic ape. Our early ancestors were primates who lived in like swamp like conditions and we evolved certain traits to deal with this. Traits like upright stances, a lack of body hair and a layer of fat under our skin (most other mammals have skin that is right over muscle).
Needless to say there's a lot of pseudo science on this, and I found a couple of interesting sites that talk about Aquatic Ape Theory, but most devolve into other ludicrous theories about the end of the world or creationism.
This site is ok, even if it does devolve into the END OF THE WORLD IN 2012.
Another good site was this site. More intelligent information.
So I've been thinking on this, and it really does make sense. Think about this on your own today.