The pilot stated that he departed visual flight rules and was returning to his home airport after work. On about a 3-mile final approach to the runway, the pilot switched from the right to the left fuel tank and when he tried to add engine power found there was no response. He then switched back to the right tank and found the engine was still producing no power. The pilot executed a forced landing in a field, which resulted in substantial damage to the wings, tail, and fuselage. An inspection of the wing fuel tanks found the left tank to be empty and dry, and a sample from the right tank revealed all water with no visible fuel. When the carburetor was drained, it had about 2 tablespoons of liquid; half fuel and half water. The pilot stated that he did not check for fuel contamination before departing. He also reported that he fueled the airplane with fuel cans that were uncapped and exposed to rain over the weekend. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.