THE PLT NOTED A PROGRESSIVE LOSS OF PWR THAT BEGAN DRG THE CLIMB TO CRUISE. HE DIVERTED BACK TOWARD THE DEP ARPT & ATMTDTO RESTORE PWR BY ACTIVATING THE BOOST PUMP, MOVING THE MIXTURE TO FULL RICH, SWITCHING FUEL TANKS & APPLYING CARB HEAT.HOWEVER, THE ENG CONTD TO LOSE PWR. AS HE MANEUVERED THE ACFT ONTO A SHORT FINAL APCH, THE ENG LOST ALL PWR & THE ACFT CRASH LNDD SHORT OF THE RWY IN A SWAMP. A POST-CRASH EXAM OF THE ENG REVEALED NO EVIDENCE OF MECHANICAL FAILURE. SVRL OUNCES OF WATER & SEDIMENT WERE FND IN THE CARB BOWL; HOWEVER, DRG THE CRASH &/OR RECOVERY OF THE ACFT, THE FUEL LINES BROKE & THE ENG BECAME NEARLY COMPLETELY SUBMERGED IN WATER. SVRL SPARK PLUGS WERE SLIGHTLY FOULED WITH BLACK, SOOTY CARBON DEPOSITS. THE ACFT WAS STC'D TO OPER ON AUTO-FUEL & WAS SVCD WITH 89 OCTANE REGULAR FUEL. THE CARB WAS EQUIPPED WITH A COMPOSITE FLOAT, WHICH HAD EVIDENCE OF MINOR DETERIORATION; HOWEVER, DRG A FLOW CHECK, THE CARB FUNCTIONED PROPERLY.