The Adventures of Billy, a Pirate's Boy

Billy falls overboard from a yacht, into the sea. His cries for help go unheard, and he uses his swimming skills to try to reach the shore. He floats on his back and falls asleep. He awakens and finds himself on the back of a dolphin that takes him close to shore. He swims the rest of the way, finds food and makes a place to sleep. When he wakes, he is surrounded by eight men, dressed as pirates, seeking a buried treasure. They take Billy with them as a pirate's boy. Thanks to Billy's encouragement, the men go to one more island and find a hidden treasure.