Re: Canada to US purchase/transfer
IF the boat was manufactured within the NAFTA zone (ie. in either Canada or the USA) then you dont need to pay import duty when you bring it to the US.
You can document it in the US before you bring it here.
You will owe tax in whatever state you bring it to as a home port. But not until you are there long enough to trigger the state's rules (could be 30 days could be more)