On March 21, 2001, approximately 1945 central standard time, a Bell 47G2 helicopter, N1575, registered to Helicopter Spraying Inc., of Ardmore, Oklahoma, and operated by Versatile Helicopters Inc., of Ardmore Oklahoma, was substantially damaged during a forced landing following a loss of tail rotor drive while maneuvering on approach to the Lake Murray State Park Airport, Overbrook, Oklahoma. The certified flight instructor (CFI) and his student pilot were not injured. Dark night visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 instructional flight. The flight originated from the Ardmore Downtown Executive Airport at 1935, and was en route to the Lake Murray State Park Airport. The CFI reported the following information to the NTSB investigator-in-charge: About 10 minutes after departing from Ardmore, the helicopter was in level flight, about 1,400 feet MSL over a lake that was adjacent to the airport. The student pilot commenced a turn to the left with a forward airspeed of about 45 to 50 knots. The CFI then "came onto the controls" to assist with the coordinated turn, and noticed that the cockpit turn indicator showed that the helicopter was "slipping." Subsequently, the CFI applied left pedal to correct, and as soon as the left pedal was applied, the airframe shuddered and started to yaw to the right. The airspeed started to decrease and the helicopter began to "spin to the right rapidly." The CFI then applied full left pedal and the helicopter continued to spin to the right. After lowering the collective and reducing the throttle, the helicopter stopped spinning. The CFI then lowered the nose and maneuvered the helicopter toward shallow water near the heavily vegetated shoreline. He then flared and pulled up on the collective to cushion the landing. Upon landing, the CFI moved the cyclic to the left to roll the helicopter in the shallow water, and he and the student egressed without difficulty. Examination of the helicopter by an FAA airworthiness inspector revealed that the splined coupling at the aft end of the forward tail rotor drive shaft segment was damaged. Removal of the dust cover for the coupling revealed that the screw and washer, part numbers AN501A10-4 and AN960-10, were loose with several of the threads stripped, thus allowing grease to weep during operation. A review of the maintenance records showed logbook entries indicating that the forward tail rotor drive shaft assembly, part number 47-644-18-9, was replaced on January 29, 2001. The helicopter had accumulated 70 hours of flight time since the last 100-hour inspection conducted on February 22, 2001, and 23 hours since the last 50-hour lubrication.