Alex-
It would be hard to find a rock to hit in
Miami or the Keys. Although they have some hard
coral and limestone type rock, it isn't quite the same as granite.(G) But you will find our NOAA
marine charts are all available online, as is a free chart viewer if you don't already have one, so that you can look at the
water depths and make up your own mind. On the trip down the coast from
Miami to the Keys, you will see that if you don't want to worry about depths, you need to take a longer
route further
offshore. And if you want a minimum of 7 feet of draft, as opposed to 8-10 feet (~2.5 meters versus nearly 3) that also restricts where you can enter. Really, the only way to say what is right for you, is for YOU to look at the
charts and see which areas will be critical to you.
Also note that in the Keys, the state patrol (FWC) is quite vigorous about enforcing anchorage rules. Areas of
coral and sea
grass where no contact with the bottom is allowed, may be a surprise to you. That should be noted on the detail charts.