The pilot was flying a night approach to a beach landing area marked by a circle of lights consisting of three motor vehicles and six flashing strobe lights. The approach was flown over water in conditions described as very dark, with no visible horizon below 500 feet altitude. The pilot reported that the approach appeared to be proceeding normally when, at what he thought was about 200 feet altitude, the helicopter suddenly yawed right and struck the water. No fault was found with the helicopter which might have contributed to the accident. Wreckage examination indicated that the helicopter contacted the water while in a right skid and that the main and tail rotors were under power at impact.