I do recall a show on PBS or Discovery Channel, where it speculated on the origin of dolphins, and indeed the thought was land mammals who lived by the sea (think sea lions or similar creatures) eventually evolved into pure sea dwellers.
This particular program speculated humans evolved from cetaceans, not apes, because of the way our arm and leg hair grows (in a streamline). Seriously.
Personally, I don't think Ann is too far off the mark. But I think it was a dog, not a bear that fell into the water. Sea lions seem a lot like dogs, and the few dolphins I have had the pleasure of meeting remind me more of a Labrador Retriever than a bear.
Evolution? Maybe. Or maybe God knew there would be seafarers someday, and they would need friendly animals around.
Of course, no doubt a big wooly mammoth fell into the ocean and evolved into the blue whale.
No, not seriously.