Allow me to sort out a few things for you:
1. If you paid sales tax on your boat in another state when you bought it, FL will look at the amount. If the sales tax paid is less than FL's...they may then want to collect the difference. If the sales tax you paid was more than FL's...they cannot collect higher.
2. You CAN use a PO Box on a drivers license. I have done so for years. You must do two things: a. Find a STATE HSMV office (not all drivers license offices in FL are run by state. Look like it, but are actually contracted offices run by county employees who are not always up to snuff on the laws) For instance, the license office in Port Charlotte (Charlotte County) does not do Hazmat testing etc, therefore they are a county office....only the state operated offices do Hazmat. Here is a list of drivers license offices statewide by county
Official Website Florida Department of Highway Safety and Motor Vehicles Just make sure its a state office and you will have no problems with a PO Box.... But you will have to provide proof of a "physical address" where you can be found. Here are the fees....
DHSMV: Motorist Services Fees Here is a link to the basic info you will have to have:
Official Website Florida Department of Highway Safety and Motor Vehicles
When you do go for a license in FL....make sure you have all of the forms, to include your slip lease, and all the rest of the documents required...or it will be a no go. Do make an appointment...saves time. If the clerk disputes the slip as an address...ask for a supervisor. Not all of the clerks are up on every aspect of the laws.
Additionally, if you own a boat in the state of FL....and it is over 30 years old, and it has its original powerplant....you can register it as an "antique",
http://www.flhsmv.gov/dmv/forms/BTR/87243.pdf The difference is large between regular and antique registrations. And "yes" even if the boat is USCG documented....you still have to register it if it will remain in FL for more than 90 days.
We are full time liveaboards....in FL, I am a lifelong resident of FL...