The flight instructor of the float equipped airplane reported that the pilot rated student completed a confined landing on a slough with minimal instructor corrections. During takeoff from the lake, the airplane accelerated onto the step, and just prior to lift off, but still below takeoff speed, the student abruptly attempted to rotate the airplane out of the water. The heels of the floats dug into the water which reduced the airspeed. The instructor took control of the airplane and put it back onto the step. Realizing that there was insufficient water remaining, the takeoff was aborted. The airplane exited the water onto wet grass and the airplane's right wing impacted the ground. The airplane sustained substantial damage to the right wing. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.