Well, I'm not much help in/around Miami. Hell, I'm probably not that great of a help for the Keys, but I was there last year and going back this year.
If you plan to wade fish, there are actually very limited access points in the Keys. Most of the flats you see on the shows were accessed by boat. Bahia Honda State Park is a nice place to go. You have to get there early, as it is a popular beach. We went, but it was later in the day and had no luck. There are a couple more spots, but I cannot remember them. We stayed in Islamorada right on the beach, so we could fish right from the house. We caught a few small barracuda, and that was it.
We went to the Bass Pro, and a nice, older gentleman told us some spots to go. He was also very informative, since this was our first time to fish the salt. Also, another great shop is the Florida Keys Outfitters
. They helped us as well, and also got us a last minute guide.
If you plan to get a guide, I would start looking now. Many of the good ones are booked far in advance and have return clients that get priority.
I have no idea of your skill level/experience, but we were completely caught off guard. We are used to fishing rivers and lakes, not the ocean. It is challenging for the beginner salt fisherman, dealing with the wind, waves, and accuracy needed to hook up.
I started a thread about it on here, Damn! That was a humbling experience.
I am getting really excited to go again. We are going down sometime in late April, and I am going to start practicing once this damn snow melts. Good luck.