The pilot of the amphibian airplane stated that the airplane was properly configured for landing on a lake, with the landing gear retracted and the flaps extended. The approach was normal; however, the pilot heard a loud bang immediately after touchdown and felt a lateral force in the cockpit. He then observed that the right wing pontoon had separated and he exited the cockpit before the airplane sank. Two witnesses were in a nearby boat and rescued the pilot. The witnesses reported that the pilot commented about the accident landing. He stated that it was only his fourth landing on the water and that the water was calm, which can affect depth perception. The witnesses added that the right wing pontoon contacted the water first, which spun the airplane. The airplane sank in an area of the lake that was approximately 120 feet deep. As of 60 days after the accident, the wreckage had not been located and recovered. The right pontoon was recovered.The pilot possessed a single-engine sea rating and reported 21 hours in the make and model airplane. He also reported 4,360 total hours of flight experience.