If you can get into John Pennekamp that would be the best bang for the buck.
If not try Key Largo Campground.
As far as snorkeling goes, there is no good snorkeling along the shore line of any of the Keys, and that includes what has been posted above. All you will see are hand full of small fish.
You need to take a dive boat out to the reef for the good snorkeling.
Don't take the ones in the state parks. They are like cattle cars.
Take a private one like Quiescence. They only take six people per boat, and will move from one reef to another to make sure you have the best experience and give you individual attention.
We have been wintering in the Keys for the past fifteen years, so I speak from experience.
With that said, Bahia Honda has the best beaches.
Jack L
Jack & Nanci