On timberwolf lake in Algonquin park we set up on an island. In the morning my partner spotted a bear track and what looked to be a freshly raked tree. In the picture perfect morning sunshine it seemed innocent enough. I applauded my pals keen eye as the track wasn't at all obvious and then we went fishing.

Later that night as we locked eyes in the tent with the realization that a bear was just outside and knocking bags around and sniffing through our tent wall we understood that Mr. Bear was living on the island. That was one unforgettable night I tell you as we both expected Mr. Bear to enter our domicile.

Last fall in Georgian Bay country I watched one swim at least 1/4 mile gap of water with ease. I don't think water is a deterrent whatsoever. If they think some food might be found on an island in the way of waterfowl nests, baby moose or frying hamburger they are coming.

As for worrying about them at night while your sleeping you might as well forget about it and go to sleep. With those big padded feet it is extremely likely you will never hear them. Rodents make more noise.   

I agree with MT and Preacher...you aren't going to hear a bear coming and I doubt you could out swim a bear.  You MIGHT be able to out maneuver one in the water.  IMO that would be crazier than wading into a group of bears swinging a stout stick ... which many of you know is MY solution.
Encountering a persistent camp robbing bear is fairly rare and dealing with one that would actually be stalking and wanting to eat YOU is less likely than being hit by lightning. 

I try to use a 5 foot stick, trying to stay out of claws' reach

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.

I should qualify my statement and say I would NEVER consider confronting a Grizzly or Polar bear with sticks and stones.  In fact, I would NEVER go camping where those kinds of bears exist.