How did the bears get from the Hydra station to the main island? Did they finally figure out a way to escape and swim for it?

I think when Dharma shut down or whatever they let them back to there natural habitat. the others dont want to have to worry about bears attacking them

Polar bears are great swimmers. The can cover a hundred of miles of open water in a day. So them getting to the other island is not so crazy. They can even get back to the Others Island with ease.