They also pounce on seals that try crawl up on the ice to sun themselves. Sometimes you can spot a bear floating around in open sea on his back eating a seal, miles from any ice, using his belly as a table. Those bears are just as comfortable in the water as they are on land. They can take a nap while floating around too.
The BS about them 'getting tired from swimming too far' is pure propaganda used to generate donations from people in California that don't know any better.