According to the pilot, he departed a hard-surfaced runway for a flight to, and touch and go landing on, a lake approximately 3 miles away. He reported that the landing was normal, and immediately on making contact with the water the airplane abruptly decelerated and nosed over. The impact resulted in substantial damage to the nose and fuselage of the airplane. Postaccident examination revealed that the landing gear was in the extended and locked position; however, the gear handle position, at the time of the accident, could not be accurately determined. According to the Sheriff's report, the pilot stated that he "forgot to put the wheels up." No preaccident mechanical malfunction or failures were found that would have precluded normal gear retraction for a water landing.