They will want to know how much you paid for it and calculate the sales tax accordingly.

Since you bought the boat from an individual in another state and assuming it has a title in another state you do have some options if you are creative. But those may not be strictly legal so won't bring them up. You'll have to figure out if you want to do something accordingly.

How much did you pay for this boat? If not a lot then the sales tax won't be much so just pay it.