According to the pilot, he taxied out, verified the amphibian airplane was configured properly, and completed all checklists. The airplane departed normally from the lake, in a southwest direction, while climbing through about 50 feet above ground level, and approaching a tree line surrounding the lake, the airplane began an uncommanded roll to the right. He then utilized full aileron and rudder authority however was unable to control the roll. The airplane continued to roll until it impacted the water. The pilot had approximately 220 hours of flight experience in the accident airplane. Post accident examination of the airplane by an inspector with the Federal Aviation Administration revealed no pre-accident mechanical problems, the wings and fuselage exhibited signs of impact damage, and flight control continuity was confirmed on all surfaces. Weather observations in the vicinity of the accident site reported winds out of the west and southwest at 10 knots or less. The pilot reported no mechanical malfunction prior to the accident.