I first went diving in Key Largo after I had dove at quite a number of places, all boat dives with divemasters. Imagine my surprise when the boat went out to the dive site, the captain gave a briefing and essentially said see you in in an hour or at 500 psi, whichever comes first.
I must admit there was some concern at that point, because this was a new ballgame for me. Fortunately, I was with a small group who had been through all this before and they put my mind at ease. I think my biggest fear was knowing where the boat was and particularly if I managed to get separated from my buddy. The most common good advice was DON'T PANIC. The boat is always (well almost always) on the surface, and if you are lost, just make a safe ascent (being very aware of any boats on the surface), find your boat (hopefully), signal you are OK and then resubmerge to swim in the direction of the boat (assuming some distance). If you don't spot your boat, any boat will hopefully do - the Keys operators are all the time picking up divers from other boats and getting them back to where they should be. Maybe a little embarassing, but not worth getting stressed out.
Some operations seem to cater to more experienced divers and some seem to be tuned in to less experienced folks. IMHO you would certainly want to use one of the latter. On my last trip to Largo I tried out two operators, Key Divers operating out of Bud & Mary's in Islamorada and Conch Republic Divers out of Tavernier. I felt that both ran excellent operations. For your trip I suggest Key Divers because they put a guide in the water with each dive group and I suspect they would even be able to supply a personal guide (for $$) if you chose to do so. Conch Republic is my choice now when I visit Largo - they are great folks, have nice boats and generally let divers dive their own plans, within the general constraints of the site that has been chosen.
Others have offered up excellent advice with regard to signals, surface marker bouys, etc. Many operations will require that you wear a snorkel. In the Keys, I have found this to be generally good practice.
I think my biggest worry would be the weather and surface conditions. The water can get pretty rough and some boats do go out even if it is fairly rough. Probably not the place to be for first time open water divers.
Good luck.
