The pilot was landing a float equipped airplane in a harbor. About 100 feet above the water, the airplane encountered a strong downdraft. The pilot applied full power, but the airplane continued to descend. The airplane contacted the water, the right float partially submerged, and the right wing struck the water. The floats separated from the fuselage, and the airplane then sank. The wind conditions in the harbor were reported to be 12 knots with gusts to 20 knots.