The pilot reported that during engine run up prior to takeoff, he was "feeling a little rushed but using my checklist." About 40 minutes into the flight, when the airplane was at an altitude of 3,500 feet mean sea level about 2 miles from shore over ocean water, the engine lost power. The pilot initiated a 180-degree turn towards a nearby airport. During descent, the pilot turned on the fuel boost pump and "got a short power surge from the engine." The pilot did not "trouble-shoot the problem much" and did not attempt to restart the engine because he was “very focused” on reaching the airport. As the airplane neared the airport, the pilot realized he did not have enough altitude to make the runway and initiated a forced landing onto a grassy field. The airplane landed hard, and the landing gear collapsed. After exiting the airplane, the pilot observed that the fuel selector was positioned to the header tank instead of the main tank. At this point, he realized that he had missed a step in the pre-takeoff checklist and failed to select the main tank before takeoff. During the 40-minute flight, the 5.5 gallons of fuel in the header tank were consumed, and the engine lost power as a result of fuel starvation. According to the pilot, the airplane sustained structural damage to the fuselage and both wings.