The pilot reported that he was descending the airplane to begin patrolling a section of pipeline when, without warning, the engine began to surge between low and high power. He diverted towards the nearest airport and began troubleshooting the issue. When it became apparent that he could not maintain altitude and would not make the diversion airport, he performed an off-airport landing in a field. During the landing the airplane struck a 3- to 4-foot-tall berm, which resulted in substantial damage to the engine mount and fuselage. During the post-accident investigation, a significant amount of water was found in the inline strainer to the engine. Follow-up interviews with the fixed base operator and pilot revealed that the pilot treated the airplane’s fuel with a 70% isopropyl alcohol (antiseptic) solution before the accident flight. The pilot was unaware that the antiseptic also contained purified water. It is likely that the airplane’s fuel became contaminated with water from the antiseptic which resulted in the loss of engine power. Review of company documentation revealed vague instructions for treating the fuel with isopropyl alcohol. For example, the instructions stated to use one bottle per 25 gallons of fuel, but did not specify the size/volume of the bottle. Specific guidance for the accident airplane was found in the manufacturer’s service information letter. The letter stated it is permissible to use any high quality isopropyl alcohol, for example one meeting Federal Specification TT-I-735a. The letter stated the isopropyl alcohol may be blended with the fuel in a concentration not greater than 1% by volume.