Mike, you really need to do some local research.
"We paid sales tax when we bought her."
In the US, "use tax" is what is levied on folks who have not paid sales tax because they bought a big ticket item out of state, or other specific situations. As documented online by your state tax authorities.
Normally if you paid sales tax you have no concern with use tax, unless you crossed state lines at some point in the process.
However, if you are in a state that did not require you to register the boat, and that normally collects sales tax only during registrations or by self-payments...If you didn't pay something that you were supposed to pay, generally the penalties, fines, and interest are levied all the way back to the date of purchase.
Check the rules for your state. Online if you can. In person or by phone if you can't find them online. And then get them documented and confirmed, don't just think some clerk on the phone really knows, because if they're wrong, you'll still owe the tax. Written confirmation is best.
If you boat the boat through a broker, usually they are required to collect and forward applicable taxes, so that all may have been done for you.