The aircraft landed in the Detroit River after the engine sputtered and the aircraft vibrated. The pilot reduced power and collective and pushed the cyclic forward. The RPM reduced from 3000 to 2500 but the rotor and engine RPM remained married. The pilot adjusted descent with collective, forward speed with cyclic, and pulled all collective just prior to impact. The aircraft was at about 150 ft AGL when the engine sputtered. One passenger described a sputtering noise but said the engine was running until impact. Another passenger described the engine noise as a muffled misfiring sound. The passengers also said the pilot did not mention the problem to them and the helicopter seemed under control until impact. Post accident examination of the engine after recovery from the water showed that the spark plugs were worn and contained a heavy build-up of carbon and lead. Near the operator's hangar were 5 sealed and 1 unsealed fuel drums with standing water on the tops. No other evidence was found to indicate any malfunction of the aircraft.