On December 26, 1999, about 1350 hours Pacific standard time, an experimental Quest II Walsh Ltd., Aventura II, N8180Q, operated by the pilot, nosed over during landing in a lake about 10 miles northeast of Antioch, California. Visual meteorological conditions prevailed at the time of the personal flight, which was being performed under the provisions of 14 CFR Part 91. The airplane was substantially damaged. The private pilot and passenger sustained minor injuries. The flight originated from Byron, California, about 1325. The pilot indicated to the National Transportation Safety Board investigator that he took off in the amphibious airplane from an asphalt runway at Byron, and then he flew to a lake near Antioch. The pilot stated that during the landing approach he forgot to retract the landing gear wheels. Upon touchdown, the airplane nosed over. The pilot also indicated that no mechanical malfunctions were experienced with the airplane during his flight.