The Florida Keys is the dive capitol of the world (or so the Welcome sign at the entrance says...lol).
Key Largo, is an eat, sleep, dive town. Marathon is a fishing and some diving town. Key West has some diving and alot of touristy activities.
There are more dive ops than gas stations here (60 in Monroe County) so there is something for everyone. Many operators participate here on Scubaboard so feel free to contact them. There are small 6 pack boats if you have a small party, there are medium sized boats, and there are big boats (usually roomy and very stable for those that could get seasick). There are shops that cater to hunters or rebreathers or tech divers. A few include a free diveguide (usually an intern), a few have their boats parked right outside a hotel, one op caters to non-English speaking Germans, French, & Spanish speakers, a couple that only sell gear. There are several career development centers.
As for Key Largo dive sites, do not miss: Molasses Reef (30+ different sites), Snapper Ledge, French Reef, North North Dry Rocks, Horseshoe Reef, and wrecks: Speigel Grove, Duane, and Benwood. In Tavernier/Islamorada, the Eagle & Alligator Reef (It'll soon be time for the Nassau Grouper aggregation there). In the middle Keys: Thunderbolt. In the lower Keys: Adolphus Bush and Looe Key. In Key West, the Vandenberg.
It all depends on what YOUR expectations & abilities are. I recommend you do a search as this is a very common topic and you will find many often praised ops, contact those ops to see if they match your expectations. I have friends at many ops and we all work hard so that you have a fun safe dive vacation and come back again.
I also second PatW's recommendation of the Blue Heron Bridge- awesome shallow dive (and it's free). You can post in the Blue Heron Bridge thread to meet up with locals who will give you a tour (it's important to know the tides there).
Please let me know if I can be of assistance, maybe even meet-up for a
:wavey: