On July 5, 1999, at 1240 central daylight time (cdt), a Cessna 180A floatplane, N9780B, operated by a private pilot, sustained substantial damage when during a precautionary landing to a lake, following partial power loss, the airplane nosed over. Visual meteorological conditions prevailed at the time of the accident. The personal flight was being conducted under 14 CFR Part 91. No flight plan was on file. The pilot and three passengers on board reported no injuries. The local flight originated at 1230 cdt, at Forest Lake, Minnesota. In his written statement, the pilot said that after takeoff from the grass strip at Forest Lake, he became concerned that his airplane was not climbing normally. He said that he could not maintain a comfortable airspeed to raise the wheels on the floats. The pilot said he became distracted with trying to maintain a flying airspeed of 85 miles per hour and an altitude of 300 feet above ground level (agl). "The plane was indicating normal temperatures and pressures. The only noticeable differences were vibrations until the propeller was dialed back to 2,500 rpm and [the] manifold pressure was pulled to 25 inches [of Mercury (HG)]." The pilot cleared his landing lane on the lake and said that he was relieved that he made his landing area. The pilot said that he did not perform a before landing check and that the landing gear (wheels) were still extended out of the floats. On contact with the water, the airplane nosed over. The pilot and passengers got out of the airplane and swam to safety as the airplane sank inverted. A Federal Aviation Administration (FAA) inspector examined the airplane after it was removed from Bald Eagle Lake. The airplane's lower engine cowling was crushed inward. The upper cowling was bent and buckled aft. The upper firewall was bent forward and down. The airplane's right side fuselage skin, just forward of the wind screen showed heavy wrinkles. The leading edge of the left wing, just inboard of the tip and running inboard to the vicinity of the left wing strut, was crushed inward. The left horizontal stabilizer was bent downward approximately 3 degrees at mid-span. The skin along the leading edge of the left horizontal stabilizer was bent and torn, upward and aft. The leading edge of the right horizontal stabilizer was bent downward and broken. The front tips of both floats were crushed aft. The wheels were fully extended on the floats. Flight control continuity was confirmed. The airplane's engine was started and run. Examination of the airplane's engine revealed no anomalies which would have resulted in the airplane's inability to gain altitude in a normal manner. Examination of the other airplane systems showed no anomalies.