THE ACCIDENT AIRPLANE HAD IN EXCESS OF 32 OUNCES OF WATER DRAINED FROM THE WING TANKS AND CARBURETOR/FUEL SYSTEM BY THE PILOT'S MECHANIC AND THE PILOT IN PREPARATION FOR A NIGHT CROSS COUNTRY FLIGHT. THE MECHANIC REPORTED HE DRAINED AND FLUSHED THE SYSTEM ACCORDING TO THE MANUFACTURER'S RECOMMENDATIONS. THE PILOT SAID HE REFUELED THE AIRPLANE WITH 88 GALLONS OF FRESH AV GAS AND WAITED 10-15 MINUTES PRIOR TO CHECKING THE WING AND BELLY SUMPS FOR WATER. HE FOUND NO WATER AND BEGAN HIS FLIGHT. APPROXIMATELY 3 HOURS LATER HE MADE A FORCED LANDING IN TREES FOLLOWING A LOSS OF ENGINE POWER. INSPECTION OF THE CARBURETOR AND FUEL SYSTEM POST ACCIDENT REVEALED SIGNIFICANT QUANTITIES OF WATER. THE FUEL TANK FROM WHICH THE AIRPLANE WAS REFUELED PRIOR TO FLIGHT WAS FOUND FREE OF SIGNIFICANT CONTAMINATES. WHEN FRESH,UNCONTAMINATED FUEL WAS FED TO THE ENGINE, IT STARTED AND RAN NORMALLY.