"I'm surprised a large live aboard state like Florida doesn't have a more formal process."
Sure they do. All the states will recognize "At sea, aboard The Ark" as an address. The problem in every state is that is neither a street address, nor a fixed address, nor a mailing address.
Every agency in every state, and the Fed, sets their own definitions for their own purposes. The Florida DMV literally sets a definition of "residence" FOR THEIR OWN PURPOSES in issuing licenses and registration, and that's how they word it.
Florida won't issue a driver's license without a POSTAL address in Florida. But there are homeless folks who won a major lawsuit some years ago and got their voting rights with a residence "Under the second bridge on the xx causeway" because that's where they lived. And up in New York, there are so many homeless folks who get their welfare and other funds sent to "General Delivery, GPO #1" that the main post office has one service window set up just for General Delivery service. Which every USPO offers.
Would Florida accept that as a residency? Well, probably not, but that's a valid mailing address.
Bear in mind, folks, that in the US you originally didn't have a right to vote at all unless you were a male LAND OWNER. If you owned land, you lived there, residency was no problem. We don't have any formal national or state policy that says you can be a resident of "somewhere that-a-way" as opposed to a fixed location, AFAIK
. You live on a boat? You're a vagrant, what else is new? Get the USPS to deliver mail to you, and you're a resident.