The flight instructor and private pilot were practicing takeoffs and landings in a float-equipped airplane. The flight instructor stated that on the seventh approach to landing, the private pilot misjudged the airplane's rate of descent, and struck the top of a spruce tree with the right elevator resulting in a full up deflection of the elevator. The airplane entered an uncontrolled descent, and impacted the water in a flat stalled condition sustaining substantial damage to the fuselage and elevator. The flight instructor stated that there were no preaccident mechanical anomalies with the airplane that would have precluded normal operation. In the recommendation section of the NTSB Accident/Incident Reporting Form 6120.1, the flight instructor stated that having personally flown with the pilot he had developed a sense of complacency, and the accident may have been avoided if he had been more vigilant, or conducted a more thorough preflight briefing.