While conducting a spray operation at 15 feet above the ground, the helicopter's engine sputtered so the pilot pulled up on the collective. The engine momentarily stopped sputtering but then began to run rough and continued to sputter. The pilot made a forced landing and the helicopter impacted the ground and rolled on its side, which resulted in substantial damage to the fuselage, tail boom, and rotor blades. A postaccident examination of the helicopter revealed water contamination in the gascolator and the fuel tank. Also, water contamination was found in the company's bulk fuel supply tank. The pilot noted that he checked the helicopter's fuel tank three times and the samples contained water every time. The pilot discarded the samples and continued to operate with the contaminated fuel.