We went to Key Largo for the first time in January and I dove with Horizon Divers. It was an awesome trip! I had only dove freshwater prior to this trip.
I suggest your bring Bonine just in case - anything about 3 ft swells gets a tad rough.
The only thing I found different about diving in the ocean was the surface was a bit choppy and I was bobbing up and down like a cork. Once you dive in, get your gear all straightened out and go down. The bobbing sensation will stop.
Also, when you come back to the boat, you may find that getting a hold of the ladder is a bit of a task with he boat bobbing up an down! Take your time, you'll get it right. Its not as bad as it appears from the water as you approach the ladder.
The Conch Fritters were good and so was the dolphin fish!
If you get to take a drive down 7 Mile Bridge, go to Key West and go to Sloppy Joe's. Great place to eat, have a beer and listen to the live entertainment. Good time place.
Your going to love Florida. I saw eels, trumpet fish - you name it. It was awesome! I would have moved there in a NY Minute! (I'm from NY!)
Dive safe and have fun - your going to love diving Florida.