IMMEDIATELY AFTER TAKING OFF FROM THE WATER, THIS PRIVATE PILOT EXECUTED A LOW ALTITUDE STEEP LEFT TURN WHICH RESULTED IN A STALL-SPIN INTO THE LAKE. NO EVIDENCE TO INDICATED PRE-ACCIDENT FAILURE OR MALFUNCTION OF THE FLIGHT CONTROL, ENGINE, OR AIRFRAME WERE NOTED. NO MEDICAL FACTORS WERE IDENTIFIED WHICH COULD BE CONSIDERED CASUAL.