THE ACFT CRASHED INTO A SWAMP WITH LOW ROTOR RPM AFTER A FAILURE OF THE ACCESSORY GEARBOX POWER OUTPUT GEAR. THE PILOT RADIOED AN EMERGENCY CALL SAYING HE WAS EXPERIENCING SEVERE VIBRATIONS AND WAS LANDING. A FEW SECONDS LATER, HE RADIOED THAT HE WAS CRASHING. RESCUE ACFT FOUND THE HELICOPTER ON ITS RIGHT SIDE PARTIALLY IN WATER. THERE WAS EVIDENCE THAT THEFAILED GEAR BOX RESULTED IN LOW ROTOR SPEED WHICH WAS NOT RECOVERED DUE TO LACK OF ALTITUDE AT THE TIME OF FAILURE. AN INVESTIGATION REVEALED THAT THE POWER TURBINE OUTPUT (PTO) GEAR, PN 4-081-044-15, FAILED FROM LOW STRESS, HIGH CYCLE FATIGUE. THE FATIGUE ORIGINATED AT A POINT WERE REHARDENING BURNS CAUSED BY GRINDING WERE PRESENT. THIS GRINDING WAS ACCOMPLISHED DURING MANUFACTURING OF THE GEAR AFTER IT HAD BEEN CARBURIZED. THE PROCEDURE SHOULD HAVE BEEN ACCOMPLISHED PRIOR TO CARBURIZATION WHEN THE MATERIAL SURFACE WAS MUCH SOFTER & LESS SUSCEPTIBLE TO GRINDING BURNS.