A Hughes 269B ditched in the ocean after an inflight loss of control. The helicopter was in cruise flight about 100 feet above the water. The pilot descended to about 20 feet above the water and decelerated to about 20 miles per hour. The helicopter began to vibrate and the main rotor RPM began to drop. The pilot turned right and increased the throttle. The pilot noticed the manifold pressure increased commensurate with the throttle application; however, the rotor RPM continued to drop. The pilot lowered the collective to regain main rotor RPM but due to the low altitude the helicopter descended into the water. The pilot reported that a tail wind existed when the helicopter experienced the loss of main rotor RPM. The winds at the local airport located 1.5 miles from the accident site were reported to be from 110 degrees at 12 knots gusting to 29 knots at the time of the accident. The pilot had accrued 75 hours in helicopters of which 40 hours were in the accident helicopter make and model and 30 hours were as pilot in command.