On June 14, 2003, approximately 1950 Pacific daylight time, a Cessna 172P floatplane, N9566L, registered to/operated by Kenmore Air Harbor, and being flown by a non single-engine sea rated private pilot, accompanied by a flight instructor, sustained substantial damage, during a loss of control on landing on Lake Washington, Seattle, Washington. Neither the student nor the instructor was injured. Visual meteorological conditions existed and no flight plan had been filed. The flight, which was instructional, was operated under 14CFR91, and originated from Kenmore Air Harbor, Seattle, Washington, approximately 1930. The instructor reported (refer to attached NTSB Form 6120.1) that the student had performed several takeoffs and landings in the floatplane into a "...direct 9 knot south wind..." and then asked to perform a westbound (crosswind) landing. He indicated that the water conditions displayed no hazardous conditions with no whitecaps or deep swells. He continued reporting: "...After configuring the airplane (Carb Heat ON, Flaps 30 degrees) and establishing standard descent attitude and airspeed (65 KIAS), the student controlled the plane throughout the landing (roundout) phase and upon direct crosswind touchdown encountered what felt to be a strong impact on the upwind float, the airplane went into a nose low attitude and spun nose over in a cartwheel motion...." The instructor reported a total of 224 hours in the Cessna 172 model aircraft of which slightly more than 6 hours were logged as instructor time (all in the Cessna 172). Additionally, his flight instructor certificate was issued on April 10, 2003. The instructor reported that there was no mechanical problem with the aircraft at the time, and winds at Boeing Field (six nautical miles south) at 1953 were reported as 160 degrees magnetic at four knots. The operator provided a training manual, "Guide to the Single-Engine Sea Rating" which contained procedures and guidance for the operation of the Cessna 172P floatplane. The manual contained a subsection under "LANDINGS" entitle "Crosswind Landing" (refer to ATTACHMENT KAH G-I). Specifically, the subsection contained procedures, which stated in part "...In the Cessna 172 floatplane, final approach speed in a crosswind landing should be 65 knots, with 20 degrees of flaps...."