On August 17, 1999, at 1609 hours Pacific daylight time, a Cessna TU 206G on floats, N206RR, sustained substantial damage when it overturned during a water landing on Clear Lake at Clear Lake, California. The private pilot/owner operated the personal flight under the provisions of 14 CFR Part 91. The pilot and one passenger were not injured. The airplane departed San Carlos, California, about 1515. Visual meteorological conditions prevailed and no flight plan was filed. The pilot stated he has a home near Clear Lake and flew there frequently. He estimated he accomplished approximately 200 landings on the lake. He said the touchdown was smooth, the airplane suddenly veered to the left, and overturned. He said the winds were light and variable and there were no white caps. A visual inspection had been completed on the floats the day before, and the pilot stated there were no mechanical problems with the airplane. The Federal Aviation Administration (FAA) accident coordinator inspected the airplane and observed a fracture and separation of the spreader bar at the front left pontoon attach point. The fracture area was corroded. He noted this area could only be observed because the pontoon was split open, exposing the attach point. The FAA inspector stated that he did not think a mechanic could normally inspect this area unless the floats were removed.