On July 17, 1995, at 1035 central daylight time, a Bell Helicopter 206B, N57536, was substantially damaged during a forced landing to the water near West Cameron 575, Gulf of Mexico. The commercial pilot was not injured. The flight was being operated by Air Logistics under Title 14 CFR Part 135 when the accident occurred. The flight originated at West Cameron 575, Gulf of Mexico, and was en route to West Cameron 587B. Visual meteorological conditions prevailed, and a company flight plan was filed. According to the operator and the Pilot/Operator Report, the helicopter was in level flight at an altitude of 500 to 700 feet when the pilot felt a high frequency vibration in the tail rotor anti torque pedals. Subsequently, a low pitch "hum" and a "shuddering vibration" was felt "followed by a loud bang from the rear of the aircraft." The aircraft began to "rotate to the right", and the application of "full left pedal" had no effect in stopping the rotation. After "several" 360 degree turns, the pilot attempted unsuccessfully to streamline the helicopter. An autorotation was initiated, and the aircraft's rotation was reduced to a "slow right motion spiral or flat spin." During the completion of the autorotation, the rate of rotation increased, and as the left float touched down on the water first, the helicopter rolled inverted. Examination of the helicopter by a Federal Aviation Administration inspector following its recovery revealed that a bolt had separated from the tail rotor aft short shaft forward flex coupling, and the drive shaft was fractured. The bolt and nut were not recovered. A hole was torn in the bottom of the engine oil tank.