I trained bottle-nosed dolphins for many years in captivity and never saw them lift a flipper to help a human being… but I wouldn’t really expect them to. They’re in captivity.
I have seen dolphins be quite aggressive towards humans in the water, especially a belligerent male looking for a mate.
Do wild dolphins actively try to help swimmers in trouble? I’ve heard those stories too, but I’ve never spoken to anyone who has witnessed it.