DURING A DAYTIME EXTENDED DOWNWIND APPROACH AT 300 FEET AGL, THE PILOT ALLOWED THE AIRSPEED TO DROP BELOW 50 MPH. THE ENGINE RPM DECREASED BELOW 1500 RPM AND SUBSEQUENTLY THE ENGINE LOST ALL POWER. THE PILOT MADE A DESCENDING RIGHT TURN FOR THE FORCED LANDING INTO A FIELD. AT APPROXIMATELY 40 FEET AGL, THE PILOT FLARED THE AIRPLANE, BUT LOST CONTROL AND IMPACTED THE TERRAIN IN A VERTICAL DESCENT. NO MECHANICAL MALFUNCTION TO FLIGHT CONTROLS OR STRUCTURAL COMPONENTS WERE FOUND DURING THE INVESTIGATION. ACCORDING TO THE MANUFACTURER'S LIMITATIONS, IMPROPER FUEL WAS IN THE AIRPLANE. THE MANUFACTURER'S FLIGHT MANUAL DID NOT ADDRESS NORMAL OR EMERGENCY PROCEDURES FOR THE AIRPLANE WHEN FLOAT EQUIPPED. THE OPERATOR WHO WAS INVOLVED IN THE MAINTENANCE OF THE AIRPLANE, REPORTED A HISTORY OF FUEL VAPORIZATION IN THE FUEL LINES.