There are retail sales and use tax exemptions (pdf) for sales of, or repairs to, watercraft and component parts that are primarily used in commercial deep sea fishing operations outside the territorial waters of Washington. This exemption is not available for watercraft and component parts used for kelping, purse seining, or gill netting, because such fishing methods can only be performed in Washington waters (the three-mile limit).
If you are regularly engaged in commercial deep sea fishing operations, you are also entitled to a sales and use tax exemption for diesel fuel used in the operation of watercraft. To be "regularly engaged" in commercial fishing operations and to qualify for these exemptions, you must earn at least $5,000 per year at this activity. (If you engage in commercial deep sea fishing and also charter boats for sport fishing outside the territorial waters of the state, you may combine your income from both activities in order to meet the $5,000 threshold.)