The commercial pilot was on a Title 14, CFR Part 91, passenger flight in conjunction with his fish guiding business when the accident occurred. The pilot reported he was flying about 700 feet above ground level in his float-equipped, single-engine airplane, when he began transferring fuel from the auxiliary fuel tank to the main fuel tank, and the engine quit. After a failed attempt to restart the engine, he said he was forced to land the airplane on the tundra. The pilot said there were no known mechanical problems with the airplane prior to the accident. He further reported that it had been raining for several days, and he should have sumped the fuel tanks prior to takeoff. He also stated that he should have initiated the fuel transfer at a higher altitude over terrain that was suitable for landing. He said the float attachments and the fuselage were damaged during the accident.