AFTER DROPPING OFF EQUIPMENT AT A LAKESIDE CABIN, THE PILOT BEGAN TAXIING THE FLOATPLANE ON WATER IN GUSTY DOWNWIND CONDITIONS IN PREPARATION FOR TAKEOFF FROM THE LAKE. WHILE FAST TAXIING (ON THE STEP), HE NOTICED THE GUSTY WIND CONDITIONS AND RETARDED THE THROTTLE. THE AIRCRAFT NOSED OVER AND CAME TO REST INVERTED IN WATER.