On January 16, 1997, about 0920 eastern standard time, a Cessna 152, N69219, registered to Boca Airport, Inc., collided with trees while making a forced landing following loss of engine power at Boca Raton Airport, Boca Raton, Florida, while on a Title 14 CFR Part 91 instructional flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft received substantial damage and the student-rated pilot received serious injury. The flight originated from Boca Raton, Florida, the same day, about 0825. The pilot stated she performed a normal preflight inspection and drained the fuel tank sumps and fuel strainer. She did not observe any water in the drained fuel. After takeoff she performed maneuvers west of Boca Raton for about 45 minutes, before returning to the airport for takeoffs and landings. On the second approach to landing she pushed in the throttle to increase engine power while at 500 feet on final approach. The engine did not respond and the propeller stopped turning shortly after this. She was unable to reach the runway and made a forced landing in a wooded area short of the runway. Postcrash examination of the aircraft by an FAA inspector showed each fuel tank was about three quarters full of 100LL fuel. Uncontaminated fuel was drained from each fuel tank sump. The fuel strainer and carburetor bowl was full of water. The right fuel cap was found to allow water to leak into it when poured over the cap. The engine fuel system was purged of water and the engine was started and operated with no evidence of mechanical malfunction. The aircraft had been fueled on January 11, 1997, and did not fly until the accident flight. During the period between the fueling and the accident flight the aircraft was parked on the ramp. On several of the days it rained. The fueling source was found to be uncontaminated and other aircraft fueled from the source did not contain water contamination.