On June 19, 1998, about 1000 eastern daylight time, a homebuilt Progressive Stingray amphibious aircraft, N6277K, registered to a private individual, operated as a 14 CFR Part 91 personal flight, crashed on landing at Lake Tarpon, Florida. Visual meteorological conditions prevailed and no flight plan was filed. The airplane was substantially damaged, and the commercial-rated pilot was not injured. The flight originated from Lake Tarpon about 10 minutes before the accident. The pilot stated that he was conducting touch-and-go landings on the lake, and during the second approach, he checked the landing gear handle to insure that it was in the up position, but he could not remember the position of the landing gear lock before or after touchdown. The airplane was landed with the gear unlocked, then nosed down in the water momentarily, and came to rest in an upright position. Examination of the landing gear by an FAA inspector revealed that the landing gear mechanical linkage was intact. All rods and linkages were connected and secured.