THE ACFT WAS DITCHED IN THE FLORIDA BAY AFTER THE ENGINE MALFUNCTIONED. ACCORDING TO THE PLT, THE ENG STARTED TO RUN ROUGH AND HE DITCHED THE ACFT AT ABOUT 0600 EDT. HOWEVER, THE ACFT CLOCK WAS STOPPED AT 0345 AND A COMMERCIAL FISHERMAN SAID HE SAW A LOW FLYING ACFT AT ABOUT 0400 HOURS GOING IN THE DIRECTION OF THE CRASH WITH NO LIGHTS ON. THE ACFT WAS FOUND IN ABOUT 4 FT OF WATER IN AN UPRIGHT POSITION WITH THE LANDING GEAR DOWN ABOUT 200 YARDS WEST OF THE CALUSA KEY. THE ACFT WAS EQUIPPED WITH AN UNAPPROVED CABIN FUEL TANK TRANSFER HOSE PLUMBED TO THE ACFT'S LEFT FUEL TANK. TWO OF THE ACFT SEATS HAD BEEN REMOVED. THE ACFT OWNERS ADDRESS ON THE REGISTRATION BELONGS TO A BUSINESS THAT RENTS MAIL BOXES TO PEOPLE WHO WISH TO REMAIN UNKNOWN. THE PLT SAID HE DID NOT KNOW THE PASSENGER; THE PASSENGER SAID HE HAD JUST MET THE PILOT. THE EXAMINATION OF THE ACFT REVEALED NO EVIDENCE OF FAILURE OR MALFUNCTION.