Basically the theory is this: for a short but evolutionarily signifigant period, the ancestors of humankind lived as a semi-aquatic species, which helped to shape our species into what we are today. The theory is widely avoided in the scientific community, mainly because when it was introduced, it was used as a feminist counter the more mainstream theory.
Some evidence supporting this theory:
1. Human behavior: Nearly all apes and other primates have a strong fear of water, while humans do not, and are quite good swimmers. Humans regularly crowd beaches in great numbers.
2. Lack of Fur: Humans have very little hair on their bodys, something very uncommon for terrestrial mammals of our size. Hair is excellent protection from the cold and from the sun... however, in water, hair does nothing to protect against the cold. Many aquatic mammals have lsot their hair, such as dolphins and manatees.
3. Skin-bonded fat: Humans, unique amung primates, have a layer of fat bonded to our skin. While uncommon amung terrestrial mammals, skin-bonded fat is an almost universal trait umungst aquatic mammals: it's vital for keeping warm underwater.
4. The human nose: Far different from the flat nose of most apes and primates, the human nose extends out and points down. While swiming, this prevents water from leaking into the nasal passage. One other primate has such a nose, the proboscus monkey... this species is semi-aquatic.
5. Human infant bodyfat: Human infants have an extremely thick layer of fat surrounding their bodies. While this does serve to help keep the small baby warm, it has a second effect... human babys float.
6. Human female breasts: The large fatty breasts of human females serve little function on land, but in water, they will float. This would allow humans to breatfeed in the water while allowing the baby to breathe.
7. Specilized bloodvessels in the lungs: Humans, interestingly, have specilized bloodvessels in the lungs which will expand at extreme depth to prevent lung implosion from the pressure. This was only a recent discovery, as these bloodvessels don't even activate until a human goes well below normal depths.
There's plenty more to the list as well: so much unexplained about human anatomy, physiology, and psycology is explained by this theory. The theory also explains how humans came to be upright walkers: in the water, we'd need to stand as straight as possible to keep our heads above water, and the water would provide support as evolution took the time to build the musculature to make the stance practical full-time. (something standard evolutionary theory canot explain)
Keep in mind, this is NOT intended to be a discussion on evolution versus creationism... this thread assumes evolution is correct.