John, I assume that you haven't spent any time ashore in Key West and Marathon. Otherwise, I think you'd certainly have your own opinion.
I can't comment on mooring ball availability, other than guessing there is a low probability of getting one within the next few months. Marathon's big attraction is sport fishing, so expect lots of powerboaters and diving charter boats to be sharing waters with you.
Key West is . . . well Key West. Expensive restaurants and high costs for provisions can be expected, as well as very "colorful" local personalities. We've always stayed in B & Bs, but hung out by the wharf district quite a bit. KW is a fun town and would probably be my choice to stay if arriving by boat.
True Blue . . .
sold the Nauticat