ABOUT 1 HR 5 MINUTES INTO THE FLT, THE PLT OBSERVED THAT THE FUEL SUPPLY WAS RAPIDLY DEPLETING. HE EXECUTED A PRECAUTIONARY LANDING IN A GRAVEL PIT & AFTER TOUCHDOWN ON THE SHALLOW WATER, THE LT PONTOON SEPARATED. EXAM OF THE ACFT REVEALED THAT THE RT WING FUEL CAP WAS NOT SECURED. FUEL STAINS AFT OF THE FUEL TANK WERE NOTED.