The airplane was at maximum gross weight and was departing a lake situated at 3,000 feet msl. After takeoff, in variable winds of 15 knots gusting to 25 knots, the pilot began to retract flaps when the airplane had accelerated to 75 mph. The pilot stated the airspeed dropped to 40 mph, the airplane stalled at about 50-60 feet agl, and descended into the water.