According to the pilot of the Champion 7KCAB, he performed a preflight inspection of the airplane, and estimated the fuel level by observing the fuel gauges, checking the fuel computer, and "sticking [his] finger in the tank." He did not visually check the amount of fuel in the fuel tanks. The pilot then departed on a banner towing flight, and after 40 minutes of flight time the engine lost total power. The pilot subsequently performed a forced landing to the water below, resulting in substantial damage to the airplane. Examination of the fuel tanks after the airplane was recovered from the water revealed that the right tank was empty, and the left tank contained about 1/8 inch of liquid spread across the bottom of the tank. Following the accident, the pilot learned that during the previous flight the fuel computer had been reset the to indicate full fuel. Following that flight, the airplane was not serviced with fuel.