Be aware that the Keys are sitting on limestone. The limestone wears in a very irregular pattern and it can be quite painful to walk on (ouch!). Many of those tiny little deserted islands are limestone coming up to impenetrable mangroves with no sandy beach at all so you can look but you can't touch. When you plan on cruising around find the places with sand so you can get off your
boat if you wish.
There are some state parks down there which may require a special
boating pass. The locals here will know about that.
Have fun