Ah! One of my favorite questions...
A question that combines two out of three of my favorite
things to do (scuba, flying, boating).
If your flying to Marathon you can dive right from that key,
you don't have to go to all the way up to Key Largo.
But if it is Key Largo that your seeking then I would fly into
Homestead X51 it is only 26 miles by car to Key Largo
versus almost 60 miles from Marathon to Key Largo.
Keep in mind that there is plenty of excellent diving from
Miami down to Marathon so you could actually fly into
Tamiami airport (KTMB) and use Florida City hotels
(cheapest rates) as your base of operations.
Unless of course you would like to try some
cavern/ spring diving then I would recomend GNV, OCF or ORL.
If you do find yourself up in Central Florida
let me know and I'll point you to some really interesting spring dives.

Webmaster/Divemaster Dave PP-ASEL
See also: www.floridascubadiver.com my website (not for profit)