The pilot reported that he felt a "kick" in the tail rotor which disappeared immediately. A short time later, the anomaly returned and the pilot elected to carry out a precautionary landing. However, the engine suddenly failed and the pilot had to use most of the rotor inertia to reach a suitable landing area. The following attempt to cushion the landing with the use of collective input was ineffective and a heavy landing ensued. An inspection of the aircraft by the operator revealed traces of water in the carburettor fuel bowl and this is believed to have caused the engine failure. No other defects or anomalies were reported.