Key Largo is the Dive Capital of the world (according to our Welcome Sign). There are more dive ops here than gas stations, so there is something for everyone from novice to experienced to tech diver and more dive sites than you could see in a week of continuous diving. Most of the dive sites are in the 25-35 ft deep range, great for new divers.
The sheer number of dive choices available can be confusing. 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 technical divers. A few have their boats parked right outside a hotel, one op caters to non-English speaking Germans, French, & Spanish speakers, and a couple of shops only sell gear. There are also several career development centers.
It all depends on what YOUR expectations & abilities are. I have dove with most of these operators and I have friends working at many of them. We all work hard so that you have a fun memorable and safe dive vacation and come back again.
Do ask your selected dive operator about accommodations as each shop has relationships with a few of the local hotels and can usually get you discounted rates or recommend a hotel walking distance to their dive boat(s).
As the others have stated, check out this Florida forum where you will find many threads about diving in the Keys with a ton of great info on dive ops, dive sites, dive training, hotels, eateries, water temps, activities, etc. Everyone has favorites for their own reasons. Search and read some threads about the dive ops that might interest you. There are many often praised shops and many who participate on Scubaboard. Feel free to call and chat with them. I am an independent instructor and work with several shops in town. Feel free to contact me if you need any assistance with customized diving or accommodation recommendations.