On September 5, 2009, about 1530 central daylight time, a Schweizer 269C helicopter, N626LH, was substantially damaged following a loss of engine power and subsequent forced landing into Lake Texoma near Preston, Texas. The pilot and the single occupant were not injured. The helicopter was registered to and operated by Longhorn Helicopters Inc., of Denton, Texas. Visual meteorological conditions prevailed and a company flight plan was filed for the 14 Code of Federal Regulations Part 91 local flight. The flight had originated from Cedar Mills Airport (3T0), Gordonville, Texas, at 1300. According to the pilot, while conducting an aerial survey approximately 350-450 feet above the water, the engine experienced a loss of power. Due to crowded beach conditions, the pilot elected to perform the forced landing off shore. The pilot and passenger were able to exit unassisted and were picked up in a personal watercraft. The helicopter was recovered from the lake and examined. Investigators removed the engine oil drain plug and drained water and oil from the engine crankcase. The engine's lower spark plugs and valve covers were removed. The engine was manually rotated via the main drive pulley and valve train continuity was established to each cylinder and to the accessory gears. In addition, thumb compression was developed in each cylinder. The magnetos were dried out and produced spark to all posts when tested via a magneto test bench. The fuel servo and injector nozzles were removed, flow checked, and disassembled for inspection. No pre existing anomalies were found. Both fuel tanks were found to contain a quantity of a blue liquid consistent with 100 low lead aviation fuel. The electric standby fuel pump switch was found in the "ON" position. The pilot reported, that per company policy, when flying at low altitude the standby pump was to be activated at all times. The pilot further verified that the standby fuel pump was on for the entire accident flight. The reason for the reported loss of engine power could not be determined.