On May 6, 2001, approximately 1345 Pacific daylight time, a float-equipped Cessna 180A, N9732B, impacted the water during a landing on the waters of Lake Washington, near Mercer Island, Washington. The commercial pilot and his two passengers were not injured, but the aircraft, which is owned by a partnership of which the pilot is a member, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which departed the waters of Roche Island, Washington, about one hour and fifteen minutes earlier, was being operated in visual meteorological conditions. No flight plan had been filed. There was no report of an ELT activation. According to the pilot, he was landing in relatively smooth water in light and variable wind conditions. Just after touchdown, the aircraft encountered a large rolling wave that the pilot had not noticed prior to landing. As a result of passing over the wave, the aircraft was thrown back into the air to a height of at least 10 feet. At that point the pilot added power in order to initiate a go-around, but the aircraft did not have sufficient airspeed to maintain flight. It therefore dropped back onto the surface of the water with sufficient force to create substantial damage in the aircraft structure. Although the pilot noticed a couple of small boat wakes when he circled the area prior to landing, he did not notice the large rolling wave/swell that he subsequently encountered. It was his opinion that the wave had been created by a large boat that had departed the area prior to his landing.