There is no
boat tax in the
USA at the national level. Each of the states has its own tax rules, making the question complex. Most states collect sales tax when a boat is purchased within state borders. Most states collect a "use" tax when a boat not purchased there stays within the state for an extended period of time, typically 90 days, or when various other criteria apply (such as a transfer of
ownership or if the boat lists a state as its home port). Ordinarily any sales and use taxes paid to another state apply as a credit towards the use tax so in effect the boat owner pays the highest rate that applies rather than the sum of the rates.
The "use" tax can apply even when the boat is foreign flagged although this varies from state to state. A cruising
permit may be necessary, and is only issued by
customs when a boat is brought into U.S. waters from elsewhere.
In
Florida the sales and use tax is 6% of the boat's
purchase price or value. There is a $18,000 maximum and there are exceptions for
boats that are immediately taken out of the state.
A boat must be owned by a U.S. citizen to be federally documented in the
USA. Many
boats on the Great Loop are state registered rather than documented. In practice there are no special difficulties taking state registered boats across the border to and from
Canada.
So, as one example, if the boat is in Florida when you
purchase it, you could register it in Florida and pay the 6% tax, and you would then be unlikely to be liable for any significant additional taxes while doing the loop -- none of the states on the loop have sales taxes above 7%, so the most they could collect is an extra 1% of the value of the boat and then only if the boat were to stay in a higher-tax jurisdiction for over 90 days.
That's a general overview. You would want to consult an attorney or tax accountant for
advice specific to your situation.