First start with the customs broker. Try to obtain the original US customs Entry Number, from the seller or previous owner, even if no duty was due the boat still had to be entered into the US. New boats built in Canada and Mexico do not have to pay import dituies along with certain other countries like Turkey, used boats are different, the customs broker can help you out. It is not likely there will be a duty.
Here in Florida we do some offshore closings, especially for non US residents, more than three miles out but less than 12 miles offshore. Three miles out is beyond Florida so sales tax is not due, but within 12 miles is still in the US. We want to stay inside the US so that any previous import duty paid such as on a trawler built in Taiwan or China remains valid. If the closing was outside the US such as the Bahamas or Canada the boat is considered as "exported" and if later it comes back to the US the duty is due again.
Having the US Customs Entry Number is important for imported boat owners when it is time to sell. It will be very hard to find on older boats, if you do not have the actaul number or date and place of entry, customs can not look it up. A few years ago I sold a Defever in Florida that was owned by a Canadian, the buyer's attorney insisted that the seller pay the US duty again, and he did pay it to insure the sale took place. It was not that expensive as the duty was based on the current value.
So try to have the Entry Number and if the duty was paid in the US do not sell the boat outside of the US if it might ever come back.