I always assumed that they didn't swim because they had such muscle mass that they sank more than humans do. It's the fat that floats. Is that incorrect? Are they less buoyant than we are?
Well, I think that is a major contributing factor... but to be honest I think it is important to remember that we aren't adapted to swim either. Humans have to learn how to swim and it doesn't come naturally. The fact that we are so good at teaching (and being taught) may allow us to engage in this activity without ever putting ourselves in too high risk of a situation (I wrote more about human/chimp teaching difference before).
For chimpanzees it is different. No active teaching really goes on at all. It is a natural reaction to be fearful of the water. Clearly, as this chimp demonstrates, they can swim. It is just getting over a deeply engrained innate fear of it coupled with the fact no one will teach them and they are poor students. Any thoughts from EvoAnth are welcomed.
Although chimps do have a large amount of muscle, it's no greater than that seen in other mammalian species that are perfectly capable of swimming if they need to. Their lack of swimming prowess seems to stem from the fact they're ability to doggy paddle (the preferred stroke of terrestrial quadrupeds) is compromised by their arboreal adaptations. Perhaps as a result of this they've lost the innate knowledge of how to doggy paddle. As such they have to learn how to swim from scratch, which is rather difficult and dangerous. The same is true of all apes, including humans. None of us have an innate knowledge of how to swim or the anatomical capacity to swim well using the typical strokes of terrestrial animals. We have to start from scratch.