The pilot departed on this third aerial application flight for the day. Shortly after take-off, the engine lost power and the pilot elected to conduct a forced landing in a field. During touchdown, the airplane landed hard and received substantial damage. After the accident, the pilot reported that this was the first flight since refueling the airplane. The pilot also added that he did not check the fuel supply, prior to refueling the airplane. An examination of the airplane and refueling tanks, after the accident, revealed water was present in the fuel.