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.