Let's assume for a moment that bears, like other large wildlife, will usually want to take the path of least resistance and prefer not to be uncomfortable. Ok, its about 80 degrees, black flies, deerflies and mosquitoes are swarming around, you weigh about 3-400lbs and you're wearing a 3" coat of dark fur. Wouldn't a nice looong swim feel really good?
I'm pretty sure bears do not consider a water crossing of any length to be an obstacle and probably relish the diversion during summer.