Your marina can be your street address. Also, commercial mailbox locations can be. St. Brendan's is the best known among boaters. There are many centers that specialize in RV'ers. Then in Atlanta there are several that specialize in pilots and traveling nurses.
St. Brendan's has one advantage by being in FL and that is it can be your physical residence, appear on your driver's license, etc., and it can be used to establish Florida residency.
One of the big questions is where do you intend to keep your boat.
If you want to re-establish residency to FL, you need to take several key formal steps that remove/renounce your previous residency. Until you do so, the previous state will still consider you to be a resident. FL does have the advantage over GA of no state income tax.