Here's what I'd do...
Take your basic open water (BOW) classes up in PA, going through a local dive shop that you can develop a relationship with. You can do your classroom and pool sessions there without pressure.
Once you complete the classroom and pool time, the local dive shop will do a "referral" to a shop in Key Largo that uses the same POI (Program of Instruction); PADI, NAUI, SSI, ... that way, when you show up in the Keys, you're already somewhat familar with terminology and equipment and then focus on "the fun stuff" while your in the Keys. In the Keys, you just do your "check out" dives and don't have to worry about using all your vacation in the classroom.
I would recommend you consider bringing dive gear with you. Keys dive shops are notorius for crappy equipment -- they get a lot of use and don't always get the service/maintenance they should. If you're flying down and baggage is going to cost -- you may have to limit your self. Diving with equipment you are familar with is half the battle.
PB | 
