The pilot of the float-equipped airplane aborted 2 takeoffs to the north from a float pond, after failing to become airborne at his no-go decision point. He said the wind was variable, and he initiated a third takeoff to the south. The airplane lifted off prior to reaching the no-go point, but he felt he was not going to clear the trees at the shoreline. He said the wind may have changed direction or quit, and the airplane was near stall airspeed. He said he had no option but to increase the pitch attitude, at which time the airplane stalled, and impacted the pond.