I did not know this until yesterday, but if you put gas in your boat, you can keep your receipts, and fill out the forms, and get back the road tax on the fuel, since your boat is not used on the road.

http://dor.mo.gov/tax/business/fuel/forms/

I estimate I would get $23.40 back if I burn 7 gallons a day and take the boat out 20 days. But I bet some on here would burn more fuel than I.