The commercial certificated pilot was landing the tundra tire-equipped, tailwheel airplane at an unimproved, off-airport site under Title 14, CFR Part 91, when the accident occurred. According to the pilot, during the base leg of the approach for landing, the engine lost power, and the airplane collided with terrain during an emergency descent/landing. The airplane's owner reported that he observed the airplane's preflight inspection, and watched the pilot sump the fuel tanks, finding a small quantity of water in the tanks. He said the airplane departed, and upon returning, during landing, the engine lost power. While inspecting the airplane after the accident, the owner said he found water in the airplane's gascolator, carburetor, and fuel lines. He said the airplane was fueled from drums with filters, and it had rained during the preceding days. He said he suspected water contaminated fuel was the cause of the engine stoppage, and that after refueling the airplane with clean fuel, the engine started and ran without any problems. The owner said the airplane received damage to its fuselage, wings, lift-struts, and landing gear during the accident.