The pilot reported that after takeoff, at approximately 200 feet above sea level, he heard a loud bang and at the same time the helicopter yawed to the right. He placed the helicopter into an autorotational descent and at the same time activated the emergency pop-out floats. This was followed by a successful forced landing on water with the helicopter remaining upright on its floats. It was later towed to the beach. The pilot's investigation revealed that the main rotor tie-down had not been stowed correctly in the rear locker by the island helicopter ground staff. The tie-down had flapped in the turbulent air beneath the main rotor and wrapped itself around the tail boom and tail rotor. The tail rotor drive shaft was sheared in the process.