On August 9, 2000, about 1210 Eastern Daylight Time, a Cessna 150, N51181, was substantially damaged during a forced landing to a field near Bridgewater, Massachusetts. The pilot-in-command (PIC) and the pilot rated passenger were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal flight conducted under 14 CFR Part 91 During a telephone interview, the PIC stated that the pilot rated passenger was a mechanic who had completed an annual inspection on the accident airplane. The mechanic wanted to take the airplane for a test flight, but did not have a current medical certificate. Therefore, the mechanic asked the PIC to accompany him on the flight. While performing a preflight inspection of the airplane, the PIC "sumped" the fuel tanks and noticed water in the fuel. He kept "sumping" the tanks, until no water was present. Approximately 20 minutes after takeoff, about 3,000 feet, the engine lost all power. The PIC performed a forced landing to a field, and the airplane came to rest inverted in a ditch. During a telephone interview, the mechanic stated that prior to the annual inspection, the airplane had not flown for approximately 3 years. During those 3 years, the fuel tanks were approximately half full. The mechanic checked the main fuel strainer during the annual inspection, and did not observe any debris or contamination. About 2 weeks before the accident flight, the mechanic completely filled the tanks with fuel. After the fueling, the engine would not start. The mechanic drained all the fuel, and refueled the tanks to capacity, but he did not recheck the main fuel strainer. After the accident, he believed that there was water in the fuel tanks that was "stirred up" during those two fuelings. Examination of the wreckage by a Federal Aviation Administration inspector revealed substantial damage to the fuselage and empennage. The inspector observed fuel contamination in the main fuel strainer and carburetor.