The Margaritaville in Key West is on Duval Street. Depending on where you dock.... if Mallory Square, it's an easy walk or if the military base (you can't walk, you are shuttled by trolleys into town) it's several blocks from where the trolley drops you downtown.
I've been to both the Margaritaville in Grand Cayman and in Ocho Rios, Jamaica. Personally I liked the one in Jamaica better, but both were good. The one in Grand Cayman is right across the street from the tender port and the pool and bar are upstairs on the roof. The one in Ocho Rios is a short distance from where we docked (about a 3 minute cab ride) but if you are not paying attention it's can be hard to find. It's in a shopping area but you wind through a long path and it's near the back on the beach. The pool at the Margaritaville in Ocho Rios had a swim up bar, the one in Grand Cayman does not. Both have slides. Enjoy your cruise!
Happy Sailing!!

Future Cruises:
Carnival Legend - September 2009
Past Cruises:
Carnival Imagination - September 2008
Carnival Legend - October 2007
Carnival Imagination - January 2007
Carnival Imagination - September 2006
Carnival Fascination - May 2005
Carnival Imagination - November 2004
Holland America Zuiderdam - May 2003