As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN CRUISE FLIGHT. VIBRATION IN AIRFRAME AND PEDALS INCREASING IN AMPLITUDE FROM SLIGHT TO EXTREME. WHEN THE VIBRATION REACHED THE EXTREME LEVEL, POWER WAS LOST TO THE ROTOR SYSTEM AND THE AIRCRAFT WAS AUTOROTATED TO THE WATER. INPUT DRIVESHAFT FAILED AND WAS FOUND COMPLETELY DECOUPLED AT THE TRANSMISSION INPUT ADAPTER.