Commercial Fishing A credit or refund may be allowed for the excise tax on fuel usedin a boat engaged in commercial fishing. Boats engaged in commercial fishing include only watercraft usedin taking, catching, processing, or transporting fish, shellfish, orother aquatic life for commercial purposes, such as selling orprocessing the catch, on a specific trip basis. They include boatsused in both fresh and salt water fishing. They do not include boatsused for both sport fishing and commercial fishing on the same trip. Caution: cheap hotels in PortoFuel used in aircraft to locate fish is not fuel used in commercialfishing.