You can register it at the boat and trailer/ATV/snow machine registry (the other one is for vehicles, don't mix them up!) in fall river.
My frame came with a BOS but he didn't sign it. The tax man showed me and said "we need a signature from the seller" as he passed me a pen and looked the other way 
You pay the tax at their computer, then walk to the office 40' away and register it.
It wasn't as much as NH but it's pretty much useless........
All our government does is distract us while they steal from us, misspend our tax $ and ruin our country