A LOSS OF TAIL ROTOR CONTROL FOR AN UNDETERMINED REASON(S). A FACTOR WHICH CONTRIBUTED TO THE ACCIDENT WAS THE PILOT'S CHOICE OF A HOVER ALTITUDE/POSITION INADEQUATE TO REACH A SHORELINE IN THE EVENT OF AN EMERGENCY. A FACTOR WHICH CONTRIBUTED TO THE SEVERITY OF THE PASSENGERS' INJURIES WAS THE OPERATOR'S FAILURE TO PROVIDE PASSENGERS WITH LIFEVESTS.