Hi jscheier,
For such a short trip I would fly to West Palm and dive in Boynton Beach/Jupiter. I have an inexpensive direct flight, car rental is easy, there are several bargin hotels, and the diving is excellent and varied. I use Underwater Explorers and Jupiter Dive Center.
If you've tried these locations in FL and are disappointed with them, a weekend in Cozumel certainly does not sound half bad and is probably one of the least expensive spots you could choose. I would generally like to stay longer than a couple days. Maybe you could return on Monday and get in an extra day of diving, that's what I usually do.
Best, Craig