The pilot stated that he and the passenger were nearing the conclusion of a local flight when the engine began to "sputter and run rough." The engine momentarily ran smooth before it began to run rough a second time, followed by a total loss of power. The pilot maneuvered the airplane for a forced landing, and stated that engine power was restored just prior to impact with trees and terrain. The airplane came to rest upright, and sustained substantial damage to both wings and the empennage. During a postaccident wreckage examination a 1-quart fuel sample was drained from the airplane, and about half of the sample was water. The pilot stated that prior to the accident flight, he had mistakenly fueled the airplane from a container of contaminated fuel.