On May 21, 1994, about 1600 eastern standard time, an unregistered experimental airplane, Challenger II, crashed while landing in Florida Bay, in the vicinity of Islamorada, Florida. The experimental airplane was operating as a 14 CFR Part 91 personal flight. The airplane sustained substantial damage. The uncertificated pilot and passenger were not injured. Visual meteorological conditions prevailed and no flight plan was filed. The flight originated from Holiday Isle, Florida, about 20 minutes before the accident. The pilot stated he was at 1,200 feet agl flying towards the coastline from the east, when he encountered turbulent air conditions over the edge of the land mass. The airplane rolled to the left, and the nose pitched down. He pushed the control stick forward, and applied right aileron and rudder. He regained control of the airplane at about 600 feet and circled back for landing. While on final approach for landing at about 75 to 150 feet agl over the same land mass, the airplane rolled to the left and the nose pitched down. He attempted the same recovery procedure as before which was uneventful. The airplane collided with the water in a nose-down attitude. He stated he could not remember what his airspeed was before the airplane rolled to the left, and that he had not determined the new stall speed for the airplane after the installation of the float kit. He further stated there was no evidence of a precrash mechanical failure of the airframe, flight controls, engine assembly or accessories.