The purpose of the flight was for the pilot to relocate the amphibious airplane from an airport with a small grass runway, to another nearby land airport. Enroute, the pilot planned to make two intermediate stops at local lakes, before proceeding to the destination. After takeoff from the departure airport, the pilot did not retract the landing gear, and climbed the airplane in preparation to land at the first intermediate destination lake, which was located about 6 miles and about 1,500 feet in elevation above the departure airport. During the climb, the pilot leaned the engine's fuel mixture, and observed four green lights illuminated on the instrument panel, after which he stated, "Gear is up for water landing." He again repeated the landing gear check with the lake of intended landing in sight; however, illumination of four blue lights on the instrument panel was the correct indication that the landing gear was retracted and configured for a water landing. While on final approach to the lake, while at an altitude of 40 feet and an airspeed of 75 mph, the landing gear warning indicator announced "gear is down for runway landing." The pilot responded by immediately initiating a go-around. The pilot pushed the throttle and propeller vernier controls forward simultaneously, but only brushed his fingers against the mixture control. The airplane continued to descend and struck the water about 10 seconds later with the engine under partial power. The airplane immediately nosed over upon touchdown, resulting in substantial damage. The pilot later was advised by witnesses who observed the accident that during the go-around attempt, that as the engine power increased, it began "sputtering." The pilot then realized that he was likely unable to hear this due to the background noise of the music he had playing and noise-cancelling headset he was wearing. Upon examining the wreckage following the accident, the pilot found that the mixture control had not been fully advanced as he had applied power during the go-around attempt. In evaluating his fitness for flight following the accident, the pilot stated, "The flight was after a stressful day of work. Mentally I was C+ material at best. Concentration and focus was not as high as I like for flying."