On July 4, 1998, about 1845 eastern daylight time, a Robert Lane Progressive Aerodyne, Inc. SeaRey, experimental, amateur built seaplane, N26RL, nosed over during a water landing on Lake Down near Windermere, Florida. The seaplane was operated by the registered owner under the provisions of Title 14 CFR Part 91, and visual flight rules. Visual meteorological conditions prevailed. A flight plan was not filed for the personal, local flight. The airline transport pilot sustained serious injuries, and the airplane was substantially damaged. The flight originated from Lake Down at 1815. The SeaRey is a single engine, high wing seaplane. It has retractable landing gear, in a conventional configuration. The fuselage of the seaplane is designed to act as a boat hull. The wings have two attached sponsons which add stability on the water. The pilot stated he conducted a preflight inspection of the seaplane, taxied, and made an uneventful takeoff from Lake Down. He then contacted a friend at the destination airport, Gator Field, who stated the weather over Gator Field included thunder and lightning. The pilot stated he then decided to return to Lake Down. The pilot stated he completed the landing checklist which included checking for a gear up light and visually ensuring the gear was up. The touchdown was smooth, and the throttle was retarded. According to the pilot, the seaplane continued normal deceleration, then it suddenly and violently dove under the water, coming to rest inverted. The pilot was unable to free himself from the seaplane, but was eventually rescued by local boaters. According to a witness, who assisted in rescuing the pilot, he was on the lake on a jet ski. He saw the seaplane was about to land, so he stopped to watch. The seaplane touched down, skipped on the water, touched down again, skipped, touched down a third time, and then nose dived under water. He stated he had seen other seaplanes land before, and everything seemed normal up until the seaplane nose dived. The witness also stated that during the landing, the gear was up. During the rescue, he stated the gear was down because he had leaned on it. Another witness concurred that the gear was up at the time of the touchdown. He stated the seaplane touched down and skimmed the water like a boat before nose diving. The FAA inspector stated he found the right main landing gear was in a position halfway between up and down, and rotated rearward. There was hull damage in the vicinity of the right main landing gear. The left main gear was in a down and locked position. The tailwheel was retracted. Examination of the landing gear extension/retraction system did not reveal any abnormalities. There were no air bubbles observed in the hydraulic system, nor was there evidence of a hydraulic fluid leak. Examination of the landing gear switch in the cockpit revealed the switch was guarded. When the gear was up, the guard was open. Slight movement of the guard resulted in the guard snapping shut and moving the toggle switch to a gear down position. According to the pilot, all SeaRey cockpits are different because they are built to the owner's specifications. The pilot decided to have the landing gear switch guarded. It was designed so a closed guard, where the guard points down, indicated down and locked gear. An open guard indicated gear up and locked. So, in order to put the gear up, the guard must be opened, a switch must be toggled, and a handle must be locked. In order to put the gear down, one could simply touch the guard because it is spring loaded to a down position. When asked, the pilot stated that no part of his body, including his head, could have touched the guard upon landing. But, he believed that the bouncing on landing could have caused the guard to close. The pilot stated he plans to either change the guard to work the other way, guard closed, indicating the gear is up, or put in a detent switch.