Interesting language of the FL DOR. I see where your situation has unclarity or conflict of law.
"The boat is purchased in another state, territory of the United States, or District of
Columbia and is brought into Florida within six (6) months of the purchase date; or
• The boat is purchased in a foreign country and is brought into Florida at any time"
The State of Florida provides exemption / waiver from use tax based on state to state reciprocity of having paid sales / use tax in another state.
It seems more details may be required to resolve your question.
Your boat has been USCG documented which implies that it has been imported into the
USA [tariff duty paid, if any was due depending on country of origin of manufacture, e.g., NAFTA exemption]. Into which State did you
import the boat after having purchased it in a foreign country [Caribbean] so as to have obtained USCG
documentation? The fact that it was imported into the USA implies it was brought into a State's waters. How long was it in that State's or States' waters? Time of stay in a State's waters triggers State registration and taxation [including, sale/use, excise, property taxation].
You stated that the boat was USCG documented with a home port in South Carolina, but was the boat actually ported in South Carolina and was it registered in South Carolina. Boats must be registered in the State of South Carolina if they remain in the state for more than 60 days. Did you pay use tax to the State of South Carolina, it would have been due upon State registration? South Carolina use tax requirements: 5% Use Tax is required when a watercraft or
outboard motor is purchased from an out of state (boat) dealer. If tax is paid to an out of state dealer, credit will be allowed.
You are required by Article 17, Section 12-36-1710, code of Laws of South Carolina to pay Casual Excise or Use tax. You must have a notarized bill of sale with the purchase
price shown to pay the Casual Excise or Use Tax.
Casual Excise Tax - The fee for casual excise tax is 5% of the purchase price with a cap of the purchase value being $10,000 and thus the tax has a maximum of $500 if the boat and
motor are purchased from same owner. When an
outboard motor is purchased as one unit, the casual excise tax is 6% of the purchase price and has no maximum fee.
If you did not pay taxes out-of-state, then you submit 5% tax to the State of South Carolina with the state registration fee.
I suspect that if you have registered in another State and paid use tax then you probably can show such to Florida and it was more than six months before entry into Florida State waters then you probably can obtain exemption from Florida use taxation based on the State to State reciprocity exemption, albeit the fine points of the law may require otherwise. I have not found the actual Florida legislation that is applicable, the FL DOR wording is likely a paraphrasing of the actual laws.