I agree, i would never say they can't swim, they are just really bad at it. There are 2 reasons. One is the body fat ratio. The second happens to be body morphology with a center of gravity that tends to propel the upper part of their bodies forward and down into the water. When we sink, we have the option of going feet first. When a chimpanzee sinks, he goes head first (shorter legs, more musclature in the shoulders). So for a chimp to swim, he has to not only expend a great deal of energy keeping his head above water, but also somehow propel himself forward without overbalancing.