On July 3, 1999, at approximately 1200 central daylight time, a Robinson R22 Beta helicopter, N8360T, was substantially damaged during a forced landing following a loss of engine power during cruise flight near Theriot, Louisiana. The commercial pilot and his passenger were not injured. The helicopter was registered to and operated by the pilot. Visual meteorological conditions prevailed for the 14 CFR Part 91 business flight for which a flight plan was not filed. The local repositioning flight originated from a farm near Theriot, Louisiana, approximately 10 minutes prior to the accident. The pilot reported that the flight was scheduled to conduct an aerial survey of alligator activity in several marshes and swamps within the local area. During the third flight of the day, at 500 feet agl, the engine of the helicopter lost power without warning. The pilot initiated a straight-in autorotation to an open area covered with 4 to 5 foot tall grass. The pilot further stated that he flared the helicopter just above the top of the tall grass, until the helicopter sank into the grass with some forward airspeed. The helicopter rolled over on its right side and came to rest heading 180 degrees from the direction of the landing. Another helicopter rescued the pilot and the passenger from the swampy area about 5 hours after the accident The 8,000-hour pilot reported that the tailboom separated from the airframe during the accident sequence and the powertrain system, including the main rotor blades, sustained structural damage. The helicopter was manufactured in 1997, and had accumulated a total of 557 flight hours. The aircraft was recovered to the helicopter manufacturer's facility in Torrance, California, for damage evaluation and repairs. On August 17, 1999, the engine of the helicopter was successfully run in a dynamometer under the supervision of an FAA inspector from the Long Beach FSDO. The inspector reported that the engine performed well within the established parameters for over 20 minutes. The inspector added that water was found in the main fuel tank, the gascolator bowl, and carburetor bowl. See enclosed manufacturer's report for details of the engine run.