The flight was enroute from Nantucket to Hyannis when the pilot heard a loud bang followed by a severe vibration, loss of power to the main rotor system, and a loss of hydraulic pressure. The helicopter made an autorotative landing in water about 25 feet in front of a fishing boat and 8 miles north of Nantucket, Massachusetts, while on a business flight. The helicopter emergency floats were at the factory being repaired. The pilot exited with minor injuries and the helicopter sank into the Atlantic Ocean. The helicopter was recovered from the water. The engine to the main rotor gearbox driveshaft assembly and a gimbal joint assembly were examined in the NTSB Materials Laboratory. The majority of the damage occurred in the area of the forward flexible coupling. The internal surfaces of the junction case had experienced severe rotational damage in the vicinity of the gimbal joint pivot points. The damage included deep circumferential channels cut into the case and severe heating/fretting.