It's not a silly question, often storing a boat can cost more than the boat.
Each locality has rules that pertain to that locality. Federal law on anchoring and mooring basically says you can anchor anywhere you do not restrict navigation. Localities, particularly in florida, have been adding in more and more restrictions on that (limiting number of days, locations etc..)
Here is a link to start you off on your search:
BoatUS.com - Tom Neale's Cruising For You
here's the nice part of that (from Florida House Bill No. 7175, Statute 327.60 (2)
"However, local governmental authorities are prohibited from regulating the anchoring outside of such mooring fields of non-live-aboard vessels in navigation.”
Unfortunately, the only way to be sure is to ask your local water cops what the rules are, start with what ever florida calls the department of natural resources (folks with jurisdiction on the water).
Simply put, no you can't just tie off to a post someone, that post belongs to someone. If you just someone elses mooring ball they can charge you for it, and most do.