If you purchase a boat from a broker in Florida, regardless of who you are, the broker must legally collect (not "charge" but "collect") the sales tax UNLESS you qualify for an exemption. In Florida, as I understand it, what would only apply if the boat was not going to be registered in Florida, and was removed from Florida within 90 days, with no extension on that time.
You will find that state taxation and registration has nothing to do with your nationality, a "boat" with a permanently installed engine is usually classed as a "motor vehicle" and subject to the same laws (registration & tax) as a car here. You will also find that after living for 30 days in many (most? all?) states you may legally become a state resident/citizen. Even if that means "resident alien".
Your best bet is to email (not telephone) or write to the Florida tax authorities and say "I am planning to" or "I may be" and present the situation to them asking about tax and other obligations. Don't rely on "some guy on the internet" or "someone I spoke to on the phone" get it in writing, from the tax authority.