At about 1407 eastern time, a float-equipped Piper J5A was damaged in a hard landing at Jack Browns Seaplane Base. The pilot was not injured. The pilot and another person had recently purchased the airplane and then installed floats. On the accident flight the pilot had been performing high-speed taxi tests and decided to take off. When the airplane was about 300 feet above the water the engine sputtered. At about 20 feet above the water, the airplane stalled and struck the water right wing low. A witness said the pilot flared too high. Postaccident examination revealed the gascolator was approximately half full of water. The engine operated normally in tests.