On August 2, 2014, about 1100 central daylight time, an Avid Amphibian, N4118X, impacted a lake after an inadvertent takeoff. The non-certificated pilot received serious injuries and the passenger was not injured. The airplane was registered to and operated by the non-certificated pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. The airplane was substantially damaged. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The non-certificated pilot reported he drove the airplane to a nearby lake to "drive it like a boat" to test the engine and cooling system. The pilot did not hold an aircraft mechanics license and the airplane was not current with airworthiness inspections. The pilot stated he "had no intent to fly." The pilot made four passes across the lake and reported the engine was operating well but was overheating; he had to turn the engine off to cool after each pass. For the last pass, the pilot input full flaps to "see if it would get the plane faster" to assist in cooling. The airplane became airborne and climbed above 100 feet. The pilot attempted to "steer" away from trees but "could not make the airplane turn." He was unable to control the airplane; he cut the throttle and impacted terrain. According to a post-accident examination by an FAA inspector, there were no mechanical anomalies with the airplane. In the pilot's written statement to the National Transportation Safety Board he noted that the accident might have been prevented if he had solicited a certificated pilot or mechanic to test the engine or he had received pilot training to be proficient when the airplane took off.