If the boat was originally built in the US or in Canada, you can import/export
it between the two countries without paying duty. (NAFTA)

Depending on the province, you might have to pay a government sales tax in
Canada- but you can apply for a refund when the boat is exported.

Once you get the boat to your home in the US, you will be subject to the same
taxes and fees that would be due if you purchased the boat in the United
States.
You will be charged a "use tax" in many states, equal to the amount of the
sales tax you would have paid if you had purchased the boat in the US.