Depends on what type of boat - if you are happy with a small catamaran type - Hobie Wave - the causeway out to Key Biscayne has a couple of rental shops - they used to rent small sailboats out of Dinner Key Marina - but destroyed by Irma - Keys might be the same - there were a couple of small rental places that rented 22' sailboats - but they may have been wiped out also.
Lots of places in Keys were damaged so availability this year is very limited - Key Largo and Key West were pretty much spared but in between a giant mess.
I've been following the recovery efforts of the Keys and they are mostly back in business for the winter tourist season. There is still a lot of storm damage debris they are cleaning up, but most tourist places are back up and running. The pool side landscaping may still be a mess in most places, but they all are working too get docks and pools repaired, cleaned and usable.
I've seen some photos of decks still broken up, and tiki bar canopies missing, but the debris has been removed and construction is underway. I've heard that the pool maintenance people are backed up trying to clean and repair all of the pools and get them filled for the winter tourist season.
There will still be demolition debris in people's private yards for a while.
The Key's businesses and residents are hoping we will all be going there soon. They need the business for their recovery.
I can only think of three options for day sailing in south Florida.
1. Go out an inlet, sail up the coast, turn around and sail back, go back in the inlet. In December this might be tricky depending on weather.
2. Sail around Biscayne Bay. Somewhat protected so less sensitive to winter fronts. Also some places to go besides going up and down and back.
3. West Coast. There are protected areas around the west coast that are options for day sails. Not exactly south Florida but Tampa Bay has lots of options. Also Ft Meyers - Charlotte Harbor area which is more south but SW.