I remember as a naive kid hearing that dolphins were known to push shipwrecked sailors to shore and telling it to my grandfather. He promptly replied; “Of course the ones they pushed the other way never got to tell their story”. Gramps was a realist.
Herodotus tells the story of how Arion was carried to safety on the back of a dolphin--he proves it by citing a statue of a man riding on a dolphin which Arion dedicated. This would have been around 600 B.C. Of course if it is in Herodotus it must be so.
Arion was supposedly the best singer in the world. The sailors on the ship he was traveling on were about to throw him overboard but they allowed him to sing before he went overboard--so it may be that he was rescued because the dolphin liked his singing.