If you are getting a loan, ask a knowledgable boat lender. Trident funding steered me around a few pitfalls and I avoided a few fees, legally.
NH exempts boat sales tax. You might have to complete the sale in NH. Mass will have some sort of 60 or 90 day rule that says no matter what, if you are here longer than that you must register and then the tax rules kick in. Lucky you to be in NH.