I can’t explain the swimming thing, that is a little weird. But, Rey spent her whole life on Jakuu having to defend herself, scavenge machinery of countless types, and I’m sure all the scavenging has given her a pretty good sense of how all the stuff works. It’s not explained outright in the movies, but it doesn’t need to be. It’s implied that she knows how to do these things because they were survival skills she needed on Jakuu. Her life before the start of A Force Awakens is a mystery, so how can we say she didn’t earn her abilities?
We don’t SEE Westley being trained by the Dread Pirate Roberts, we just take his word for it that he somehow has an immunity to the poison.
I’d like to point out that Finn ALSO uses the lightsaber without any training, but he doesn’t get any flack for that.