According to US press, the 66-year-old man was hit while diving in about 3m of water at Pickles Reef. The boat did not stop.
The diver's two brothers pulled him aboard their private craft, took him ashore and, after attention at a local hospital, he was flown by helicopter to a major hospital for further treatment. He was reported to have sustained serious lacerations to his lower legs from the boat's twin props.
The area's Fish and Wildlife Conservation Commission has been attempting to trace the boat involved, whose helmsman may not have realised that he had hit anyone. The boat was described as a centre-consoled vessel of about 9m in length.
The diver's brothers stated that they were flying a divers-down flag and estimated that he was about 30m away when he was hit. Local regulations stipulate that vessels should pass at very low speed if passing within 90m of a divers-down flag.