MJ...you are responding to an old thread...but to answer your question...federally documenting your boat does NOT relieve you from the obligation to REGISTER your boat OR pay sales and/or use tax on it. Some states do not have these taxes...but most do.
The only thing federal documentation permits you NOT to do is putting state numbers on the bow of your boat.
If you are in Provincetown Mass as per your header than hyou OWE sales/use tax to Massachusetts. Don't believe me...then have a look at this.
Pay particular attention to step#2 where they ask for your federal documentation as proof of ownership!
As near as I can figure...you owe the state of Massachusetts 5% of your boat purchase price + an $80 registration fee.
If you keep the boat out of state your costs may vary!