The pilot of the amphibious airplane was performing numerous touch and go water landings to a lake. During the last landing, to the south, the airplane touched down with the nose a "little bit" to the right, the left sponson then hit the water, and the airplane water looped. Several minutes later, the airplane sank to the bottom of the lake. The airplane incurred substantial damage to both wings and the fuselage. The pilot reported that there were no mechanical malfunctions or failures of the airplane prior to the accident. A weather observation taken about the time of the accident, and approximately 25 miles north of the accident site, included winds from 180 degrees true at 8 knots.