What happened
During a training flight, a student pilot was climbing through 400 feet above the water in a float-equipped airplane. To conduct a simulated engine-out forced landing on the waters of a bay, the instructor pilot reduced power to idle and instructed the student to execute the maneuver.
The student established the aircraft in a controlled slip to manage the descent toward the intended touchdown point. When the aircraft reached approximately 50 feet above the water while still in the slip, the instructor—having just cautioned the student regarding airspeed—perceived that the aircraft was approaching a stall. To prevent this perceived stall, the instructor applied full power and pushed the control stick forward.
As a result of these inputs, the aircraft pitched down and impacted the water surface at a 30 to 40 degree nose-down attitude. The student pilot believed the aircraft was not actually entering a stall, but rather that the instructor had misinterpreted light turbulence and the wing-low attitude of the slip as an impending stall.