The pilot stated that he was towing a banner along a beach when the engine gradually lost power, and applying carburetor heat had no effect. A forced landing was initiated on the beach, and during the landing roll the airplane struck a person before it nosed over into the surf. The engine mount, right wing, and right rear lift strut were substantially damaged. Postaccident examination on site revealed that the fuel selector valve was in the left tank position. A follow up examination determined that no fuel was visible in the left fuel tank site gauge inside the cockpit area. The right wing filler cap was removed, and fuel was observed in the right tank. Fuel drained from the sump at the rear of the right tank, but nothing drained from the sump at the rear of the left tank. No fuel drained from the gascolator on the firewall. The operator reported that there were no mechanical malfunctions or failures of the airframe or engine that would have precluded normal operation.