On June 27, 1997, about 0730 Alaska daylight time, a float equipped PA-18-180 airplane, N3685Z, sustained substantial damage while landing at Moon Lake, located about 20 miles west of Tok, Alaska. The airline transport/certificated flight instructor pilot-in-command (PIC), and the commercial certificated second pilot, reported no injuries. The U.S. Government flight was operated by the U.S. Fish and Wildlife Service under 14 CFR Part 91 as an instructional flight. The purpose of the flight was for the flight instructor to assist the second pilot in qualifying for PIC flight status on float equipped airplanes with the U.S. Fish and Wildlife Service. The second pilot was rated for float planes, but did not meet the U.S. Fish and Wildlife's criteria for PIC. The local flight operated in visual meteorological conditions, and a company VFR flight plan was in effect. According to an Office of Aircraft Services investigator who interviewed both pilots, the second pilot was flying the airplane on an approach to Moon Lake to practice glassy water landings. The accident landing was the fourth or fifth touch and go landing at the same lake. On the accident landing, the airplane momentarily touched down short of the lake in a flat, grassy area. Engine power was applied, and the airplane continued a short distance to an uneventful landing on Moon Lake. Postflight inspection of the airplane by another flight crew disclosed two damaged longerons near the left float's rear attachment point. The PIC noted in his written report to the NTSB that on the accident approach to land, the airplane possibly encountered a downdraft just prior to the inadvertent touchdown in the grassy area.