If you have a Washington-registered watercraft, you can apply for a fuel tax refund account, and then submit refund claims for Washington fuel tax that was paid on fuel for your boat. Currently the tax is $0.375 per gallon, and you need to accumulate at least 54 gallons in fuel purchases per refund claim you file (doesn't have to be all in one purchase).

In 2 days of riding, I think I've burned 25 gallons of fuel, so this could add up to a decent refund over the course of a season. Of course it requires keeping proper records and filing some tax forms, but sometimes every little $ counts!

The refund claim applies to all non-highway use, so fuel used in off-road motorcycles, ATVs, tractors, lawnmowers, etc. should be includable.