The pilot of the float-equipped airplane reported that, following takeoff from a lake, he turned to left downwind about 750 ft above the water. He added that, once the turn was completed, the airplane encountered a strong downdraft, which "pushed [the] tail of [the airplane] down and away to [the] left," and the airplane began to descend. The pilot corrected with left rudder and was able to "straighten [the airplane] just prior to [the] right float tip contacting water." The airplane impacted the water, nosed over, and sank. The pilot egressed without further incident. The airplane sustained substantial damage to both wings. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. The pilot reported on the NTSB Form 6120.1 that the wind was from the east/southeast at 15 to 20 knots, gusting to 25 knots. The pilot reported a takeoff to the east. A review of recorded data from an automated weather observation station located about 7 nautical miles east of the accident, about the time of the accident, revealed winds calm; 10 statues miles visibility; few clouds at 600 ft, scattered clouds at 1,200 ft, and an overcast ceiling at 3,200 ft; temperature 54°F; dew point 54°F; and an altimeter setting at 30.19 inches of mercury. The computed density altitude at 750 ft was 322 ft. The Federal Aviation Administration's Airport Facility Directory for an airport 2 nautical miles west of the accident site stated, in part: "strong East winds, rwy subj to strong downdrafts."