The loss of engine power due to the pilot's inadequate preflight, which failed to detect water contamination of the fuel system. The operator's inadequate maintenance of the loose fitting fuel cap and the deformed filler opening, which allowed water to leak into the fuel tank, were also causal. A factor in the accident was the trees along the autorotative flight path, which required the pilot to stretch the glide and resulted in a low main rotor rpm situation for the landing flare.