A fascinating example of this are some Labrador retrievers. Labs are descended from a Newfoundland "landrace" of dogs known as St Johns Water Dogs. They have multiple aquatic adaptations: the "otter tail", oily fur, and webbed feet. (Some of these are shared with other water-oriented breeds.) Some lines of Labradors, especially the "bench" or English dogs, normally retain this full suite of water adaptations.
But the wild thing about these particular Labradors is that they love to swim, and that most of them are born knowing how to swim very well. But they don't know that they know how to swim. So many a young Lab will spend a while standing on the shore, watching humans or other dogs in the water, and fussing because they don't dare to join the fun. Then they may (for example) eventually lean too far and fall into shallow water. Within moments, they'll typically be swimming around and having the time of their lives.
The near-instant transformation from "fascinated by water and fearing it" to "hey I can swim and this is the absolute best thing ever" is remarkable to watch, though not recommended.
I remember another Lab, who'd been afraid to go swimming, who one day impulsively bolted for the water, took an impressive leap off a rock, and (from his reaction) apparently realized in mid-air that he had no idea what he was going to do next. Once he hit the water, he was fortunately fine, to the great relief of his owner.
CAUTION: This behavior pattern is apparently NOT universal in Labs. Owners of "field" or American Labs seem to have much better thought-out protocols for introducing hunting dogs to water, and failure to follow these protocols may result in bad experiences, dogs that fear water, and actual danger to dogs. So please consult an expert.