If as stated, NH does not have a sales tax, Ma does, you should be able to do as OR residents do when buying Wa st, and that is have a form/card that shows you are a non resident, you do not pay the tax, get the boat home, and pay appropriate NH tax. Or also as mentioned, if both have STax,, you pay or collect back the different once in your home state.
Just look things up tomorrow with you state tax agency to find out the rules. And as seawolf says, and you seem to know, the tax man will getcha! So plan to see him before he comes looking for you!
She drives me boat,
I drives me dinghy!