THE STU PILOT AND HIS STU PILOT RATED PASSENGER WERE FLYING LOW OVER THE ARKANSAS RIVER AS OBS BY TWO EYEWITNESSES LOCATED IN A BOAT IN A COVE NEAR THE EAST BANK OF THE RIVER. THE ACFT DISAPPEARED FOR A BRIEF TIME UNTIL IT AGAIN WAS SPOTTED, APPROACHING DIRECTLY AT THEM AT LOW ALTITUDE FROM THE NORTHEAST ALONG THE FINGER COVE. THE EYEWITNESSES STATED THE ACFT FLEW DIRECTLY OVERHEAD AND PULLED UP SHARPLY INTO A CLIMB. AT THE TOP OF THE CLIMB THE RIGHT WING STALLED AND THE AIRCRAFT DESCENDED RAPIDLY NOSE DOWN UNTIL HITTING THE WATER. THE AIRCRAFT RAPIDLY SANK IN 30 FEET OF WATER BEFORE IT COULD BE REACHED. THE STU HAD RECENTLY BEEN RELEASED FOR SOLO TRNG FLTS AND HAD ASKED IF PASSENGERS WERE ALLOWED. THE STU WAS INSTRUCTED THAT NO PASSENGERS COULD BE ALONG. INSPECTION OF THE ACFT REVEALED NO PRE-EXISTING MECHANICAL DEFECT OF FLIGHT CONTROLS OR AIRCRAFT ENG COMPONENTS.