All other species of ape are afraid of water because their muscles are more dense, less fat, more fiber. The result is that they have extreme difficulty floating. I've also read they're less pneumonia resistant so if they get water in their lungs they're fucked. Not sure if that's true or not. One of the theories for how modern man evolved is that we did so somewhere with a lot of shallow water, where being more buoyant and bipedal would be a huge advantage.