The pilot reported that during the initial climb out for the aerial application flight, he noticed a power loss and initiated an autorotation into a cornfield. During the landing, the fuselage, tail rotor and horizontal/vertical stabilizers were substantially damaged. Fuel drained from the helicopter's gascolator during recovery contained a large quantity of water. Fuel storage for the spray operation consisted of a primary (filtered) and a secondary (unfiltered) storage tank. When fuel was needed from the secondary storage tank, it would be transferred to the primary storage tank to ensure filtering. On the evening prior to the accident, over 4 inches of rainfall occurred at the airport. The operator thought a fuel cap may have been left off the secondary storage tank inadvertently, which may have allowed water to enter the tank. On the day of the accident, due to a pump failure in the primary storage tank, fuel was drawn directly from the secondary (unfiltered) storage tank. The operator stated that rushing contributed to loading unfiltered fuel into the helicopter. The pilot did not sump the helicopter's fuel tanks as required by the flight manual during preflight and/or after refueling, which would have identified the water contamination.