You should be able to get into No Name Harbour at the southern end of Key Biscayne, but I'd double check with the Crandon Park rangers to verify the depth
is adequate (shoaling). The best anchorages
are on the north side of Watson Island, just off the MacArthur Causeway and off Dinner Key in Coconut Grove. Stay away from Miami beach as they're notoriously rude to boaters and have passed laws limiting anchoring
If you need more info on how to get to the two anchorages, let me know.
Good Luck and welcome to Miami.