I live in Coconut Grove which is at the top of Biscayne Bay and 3-4 miles south of where you intended to anchor. My boat
is presently out in the free public anchorage off Dinner Key Marine
in Coconut Grove. This gives excellent access to shops, fuel
and restaurants. The Coconut Grove Sailing Club offers guest moorings for $30/day up to 40 ft which is a much safer place to leave the boat if you are going to be away.
However, cruisers in the anchorage tend to take care of each other and watch each other's boats. Holding ok but not great in anything over 30 knots...most people have 2 anchors down