AFTER TAKEOFF, THE AIRCREW FLEW TO THE SOUTHERN END OF LAKE WINNEBAGO WHERE THEY MADE SEVERAL UNEVENTFUL WATER LANDINGS. THEY ESTIMATED THAT THE WIND WAS FROM THE NORTHEAST AT 5 TO 6 KTS. THE WATER WAS DESCRIBED AS SMOOTH WITH ONLY SMALL RIPPLES. ON THE 6TH APCH, THE GEAR HANDLE WAS CHECKED IN THE UP POSITION & THE AMBER GEAR UP INDICATOR LIGHT WAS ILLUMINATED. THE AMPHIBIOUS ACFT TOUCHED DOWN ON THE WATER WITH NO BOUNCE OR PORPOISE. ABOUT 2 OR 3 SECONDS AFTER TOUCHDOWN, THE ACFT YAWED VIOLENTLY TO THE RIGHT & ROLLED LEFT. THE LEFT FLOAT & WING TIP HIT THE WATER & THE LEFT WING SEPARATED. THE HULL ROLLED OVER, BUT THE OCCUPANTS EGRESSED BEFORE THE ACFT SANK. A DIVER NOTED THAT THE RIGHT GEAR WAS EXTENDED. AFTER THE ACFT WAS RECOVERED, THE RIGHT MAIN GEAR ACTUATOR ROD CLEVIS WAS FOUND TO HAVE FRACTURED. THE FRACTURE SURFACE HAD SIGNS OF OVERLOAD FAILURE. THE LOCK NUT HAD BEEN SCREWED TIGHTLY AGAINST THE CLEVIS & THE LOCK NUT THREADS HAD PRODUCED GOUGE MARKS ON THE UNTHREADED PORTION OF THE SHANK OF THE CLEVIS.