On August 1, 2004, about 1030 central daylight time, a Lake LA4-200 amphibian, piloted by a commercial pilot, sustained substantial damage during a water takeoff from Lake Winnebago near Oshkosh, Wisconsin. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions without a flight plan. There were no injuries to the pilot or passenger. The flight had originally departed from Cushing Field Airport near Newark, Illinois, about 0900, and was bound for the Seaplane Base at the annual Experimental Aircraft Association gathering in Oshkosh, Wisconsin. The pilot reported that he landed mistook a harbor for the seaplane base and landed. He said that after he realized that he was north of the seaplane base, he attempted to takeoff. He said that during the water takeoff, he encountered a large wave that "launched" the airplane into the air prior to achieving takeoff speed. He said that the airplane was damaged when the airplane came down and impacted the water.