On August 9, 1997, at 1335 hours Pacific daylight time, a Cessna 150G, N3084S, lost engine power and the pilot subsequently ditched the aircraft in the Pacific Ocean off Point Vicentes, Palos Verdes, California. The airplane, owned and operated by Tom King Aviation, sustained substantial damage. The pilot was not injured. Visual meteorological conditions existed for the banner towing operation. The flight originated from the Chino, California, airport at 1145, and no flight plan was filed. Review of the aircraft records disclosed that it had been modified, in part, with installation of: 1) a Lycoming O-360 engine; 2) an 18-gallon auxiliary fuel tank in the baggage compartment; and 3) a Cessna 172 type main fuel selector capable of selecting right, left, both, and off positions. According to the Federal Aviation Administration (FAA) Form 337 (Major Repair and Alteration), the auxiliary tank was plumbed to pump fuel through an electric pump and an on-off fuel valve into the right wing tank. The fuel quantity sending unit was wired through a toggle switch to the right wing tank fuel gage; the switch could select either the right wing or auxiliary tank for gage display. Review of the system operating procedures specified in Form 337 disclosed that the pilot is to use fuel from the right wing tank first for 1 hour, then transfer fuel from the auxiliary tank. The pilot reported that after picking up the banner he climbed on a westerly heading on both tanks. Approximately 15 minutes later the pilot switched to the right tank. He reported that 1 1/2 hours later he switched to the auxiliary tank. The right tank fuel gage read 1/4 full. At this point the engine stopped, and he switched back to both tanks in an attempt to restart the engine. The pilot reported that the attempted engine restart was unsuccessful and he had to ditch the aircraft in the ocean. The pilot stated that after the aircraft was recovered, 2 gallons of fuel were drained from right tank, and 18-20 gallons of fuel were drained from the left tank. The pilot stated that in his opinion the reason the engine stopped was due to, ". . . fuel exhaustion of useable fuel in right tank." In a verbal statement to FAA inspectors, the pilot reported that he forgot to switch the fuel selector. Following recovery of the aircraft, the engine was cleaned up, then started for a test run. No mechanical anomalies were noted with the aircraft or engine.