Do not know specifically about the regulations
in WA but most states, if you buy a boat there but are not a resident of that state, AND you move it out of that state withing a deadline (usually 30 days) THEN you will be liable for the tax, if any, in the state where the boat is docked and/or registered.
So if OR has no sales tax on a boat and you keep the boat there, make that your legal
residence and register the boat there then you should be clear of any sales tax. You might want to check on any potential for state income
tax or property tax.