After a normal preflight inspection and takeoff, the pilot selected the landing gear up. The indicators were later checked and the pilot found that all the up lights were indicating normally, but one down light was also on. During the short flight the pilot operated the landing gear a number of times. He was satisfied throughout the flight that the landing gear was retracting; his only concern being whether the gear would extend again on his return flight. During the gear checking the aircraft arrived in the circuit area of the destination. The pilot made routine radio calls to his company and Townsville Flight Service, checked for boats in the landing area and noted that there were only four landing gear lights illuminated. On touchdown the pilot was unable to prevent the aircraft from pitching excessively nose down. The aircraft came to rest with the engine and some of the wings in the water. Subsequent actions by the pilot and a nearby boat resulted in the aircraft being righted and then towed to a mooring site. The landing gear had been in the down position for the landing. Since the landing gear indicating system uses four blue lights to indicate gear up and four green lights to indicate gear down, the pilot concedes that he had not noted the colour of the lights adequately in order to determine whether the gear was up or down.