On July 17, 1999, at 1723 central daylight time, a Piper J-3C, N88593, was substantially damaged when the float equipped airplane departed controlled flight during the takeoff climb and impacted the water. The private pilot and passenger received serious injuries. The 14 CFR Part 91 flight was departing from Maple Lake, Minnesota, on a local flight. Visual meteorological conditions prevailed and no flight plan was filed. A witness reported the pilot had departed Lake Charlotte near Buffalo, Minnesota, and landed on Maple Lake, near Maple Lake, Minnesota. The witness reported the flight was normal and uneventful. The witness reported that a neighbor of the pilot boarded the airplane for a ride. The witness reported that during takeoff climb, the airplane climbed above treetop level, and then "... abruptly returned to the surface." A Federal Aviation Administration Aviation Safety Inspector reported that he had interviewed the pilot. The FAA investigator reported that during the course of the interview, the pilot stated that he gave the passenger a ride in the airplane and that the passenger weighed over 200 pounds. The pilot stated that he did not do a weight and balance or a performance calculation. He reported the right fuel tank was full and the left tank was just below one half full. Each tank held 13.5 gallons when full. The pilot stated the airplane takeoff was to the north. He reported the airplane had pitch problems during the climb and that the nose always wanted to pitch up. He reported that at approximately 50 to 100 feet, the airplane started turning to the left and the nose pitched down, followed by a crash in the lake. The airplane was found with the elevator trim in the full down position. The private pilot had approximately 400 total flight hours with 300 hours in type, of which 150 to 175 flight hours were on floats. He had flown about 6 to 8 hours in the float airplane in the last 90 days. The weight and balance of the airplane dated May 12, 1973, recorded the maximum gross weight of the airplane as 1,294 pounds. The weights listed were: Aircraft empty weight 868 lbs. Pilot 170 lbs. Passenger 170 lbs. Fuel 66 lbs. Baggage 20 lbs. Total 1294 lbs. The approximate weight of the airplane during the accident flight is listed below: Aircraft empty weight 868 lbs. Pilot 191 lbs. Passenger 235 lbs. (Weight listed on Driver's license) Fuel left tank 6.75 gal 40 lbs. Fuel right tank 13.5 gal 81 lbs. Baggage 5 lbs. Total 1420 lbs.