A few answers here:
First, you can list a marina as your address. I have done it several times. I don't generally list it as Slip #. I generally just use #6 (for slip 6) ETC. For those who are living aboard in hurricane prone areas, I would suggest you find a way to get a local address (marina for example) of the area you are staying in. After a hurricane, they may not let you back in unless you have that address on your driver's license. This happened to us after Gabrielle and Charlie.
Second, for those who are mobile or on the hook, you can use St. Brendan's Isle. It is a fabulous service that helps in all manners of mail and setting up an address. It is completely legal and you will be shocked at the number of cruisers who use this service. Their phone number is 904-284-1200. You will need to present two pieces of mail IIRC to change your address over if you are moving in from out of state. SImply have your phone bill and/or CC sent there and that will be the proof you will need.
Third, if you are already a Florida resident, you can change your address online. No need to present two different pieces of mail or any of that. $25 and you are done. Your mailing address and the address of where you reside can be two different things. The address to do that is here: https://services.flhsmv.gov/virtualoffice/Lobby.aspx