Beavers, with their wide flat tails, are very good swimmers. Looking it up though it seems black bears are the fastest overall although I believe beavers are the fastest relative to their body size.
They're terrestrial animals first and foremost, they spend most of their lives above and out of the water, unlike dolphins and whales, which are truly aquatic. That beavers choose to live near water is hardly unique. Most animals need to live near water. I understand they're highly adapted for river life, but they move and forage on land.
