The pilot of the amphibious airplane reported that, after landing on the water he taxied to a boat ramp where he drained water out of the airplane before he took off from the top of the boat ramp. Once airborne, he established a climb attitude, but the airplane was not able to maintain a climb. He further added that there was insufficient time to raise the landing gear before the airplane landed on the water. The airplane sustained substantial damage to the right wing and aft fuselage and empennage. A witness reported that he assisted the pilot in pulling the airplane out of the water and onto the boat ramp. The water was drained out of the airplane, and then the pilot commenced with the takeoff. The witness further reported that the airplane struck a tree branch before it impacted the water. Review of recorded video provided by the witness showed that the airplane departed from the top of a declining concrete boat ramp and became airborne just before the waterline at the bottom of the ramp. Throughout the takeoff sequence, the engine could be heard with no noticeable changes in sound. The video showed the airplane briefly remained airborne at a low altitude before it descended into the water. A Federal Aviation Administration inspector interviewed the pilot. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.