THE ACFT CRASHED AFTER TAKEOFF WHEN THE ENGINE LOST POWER. THE ACFT HAD NOT BEEN FLOWN FOR SEVERAL MONTHS. AN INSTRUCTORWAS INVITED ON THE EXTENDED CROSS-COUNTRY TO HELP THE PILOT WHO ALSO HAD NOT FLOWN FOR SOME TIME. DURING THE PRE-FLIGHT A CONSIDERABLE AMOUNT OF WATER WAS DRAINED FROM THE LEFT FUEL TANK. THE MECHANIC WHO HAD JUST RECENTLY COMPLETED AN ANNUAL INSPECTION ON THE ACFT WAS CALLED OVER TO HELP DRAIN THE WATER. THE MECHANIC DRAINED 15 GALLONS OF FUEL BEFORE THE FUEL WAS FREE OF WATER. THE LEFT TANK CAP ALSO WAS FOUND TO NEED A NEW SEAL AND WAS TAPED UNTIL THEY COULD GET A SEAL. THE ACFT HAD TO BE JUMP-STARTED. AFTER STARTING THEY DECIDED TO USE ALL 6000 FT OF RUNWAY 22 FOR TAKEOFF. DURING TAKEOFF THE ENGINE BEGAN TO CUT OUT WHEN IT REACHED AN ALTITUDE OF 100-200 FEET IN THE AIR, REGAINED POWER MOMENTARILY AND STOPPED. THE ACFT TURNED LEFT, LANDED ON A SLOPING FIELD AND CAREENED DOWN INTO A CREEK BED. THE ACFT CAME TO REST ON ITS SIDE HALF SUBMERGED. TWO OCCUPANTS WERE EJECTED. ONE OF THE TWO DROWNED IN THE CREEK.