Wendy and Larry,
Be sure to bring atomic strength bug repellent, especially after dark. If you have cats or small dogs, stake them down or the skeeters will haul them off.
I used a dive shop on Marathon Key (don't remember the name), and the Reef Raiders dive shop on Key West. Of course, this was many years ago, so things may have changed. There were State parks and/or cmmercial RV parks on nearly all the bigger Keys when I was there.
I had only pleasant experiences, except for the monster mosquitoes.
I didn't dive the underwater state park at Key Largo, but friends tell me it is really nice.
2006 T-3075 Toy Hauler
2006 Dodge Ram diesel 2500 4-door