On July 9, 1994, about 1250 eastern daylight time, a Robinson R22 Alpha, N22431, registered to Palm Beach Helicopters, Inc., made a forced landing in the St. Johns River near Sanford, Florida, while on a 14 CFR Part 91 aerial observation flight. Visual meteorological conditions prevailed at the time and no flight was filed. The aircraft received substantial damage and the commercial-rated pilot and one passenger were not injured. The flight originated at Sanford, Florida, on July 9, 1994, about 1240. The pilot stated that while maneuvering at about 15 feet above the river to spot alligators, the helicopter began to spin to the right. He lowered the collective pitch control and lowered the nose thinking that the wind was causing the spin. This had no affect and he had to ditch the helicopter in the river. A witness on a boat told him that the tailrotor had stopped before the helicopter ditched. Postcrash examination of the helicopter by a FAA inspector indicated the tailrotor driveshaft was separated near the last coupling before the tailrotor gearbox. One tailrotor blade had a 90-degree bend in it across the blade face and the other blade was not damaged. The tailrotor gearbox turned freely. Metallurgical examination of the separated tailrotor driveshaft was conducted by Mr. Joe Epperson, Senior Metallurgist, NTSB, Washington, D.C. The shaft separated due to torsional overload in the normal direction of rotation. There was no evidence of preexisting cracks or abnormalities. See attached metallurgist's factual report. Logbook records indicate the tailrotor driveshaft was new when it was installed by Robinson Helicopter during helicopter overhaul on June 22, 1992, at aircraft total time 1,997 hours. The shaft had accumulated 887 total flight hours since new at the time of the accident. Logbook records indicated the helicopter received a overspeed inspection on November 17, 1993, at total aircraft time 2,637. On December 1, 1993, the lower vertical stabilizer was replaced due to ground damage. On June 21, 1994, at aircraft time 2,851 hours, the horizontal stabilizer was changed due to ground damage. See attached logbook pages.