That's great advice Debby gave you, especially if your certification dives were completed in an enviroment different from the Keys.
Regardless of what shop you decide to dive with or whether or not you dive with a Divemaster guiding you, just remember your training, don't be afraid to ask questions of the staff on board the boats and TAKE YOUR TIME!
Remember your training: I'm sure you were taught and had the chance to practice all the fundamentals throughout your checkout dives. That is the stuff that will get you out of a bind, so go over it all in your mind, each time you go diving, until it becomes second nature, which it eventually will if you dive often enough. Remember, practice doesn't make perfect, rather, "perfect practice makes perfect!" So complete your prep and actual diving the way your instructor taught you, unless the enviroment is different, then it is usually best to rely on local divemasters to show you the ropes, as Debby so wisely suggested.
Don't be afraid to ask questions: I have been actively diving for over 20 years and am rated as an PADI Open Water Instructor, Mixed Gas and Closed-Circuit Rebreather Diver, and I still have to ask questions almost every dive. When the staff gives briefings, make sure to pay close attention. If there's anything you didn't hear or would like clarified (i.e., when to suit up, when to jump in, which direction the current is running, which way to go, how long to stay down, how to safely exit the water, etc.), just ask - Don't be shy or embarrassed! Better to be safe then sorry!!
Take your time: When you are new to diving, everything will take you longer to accomplish, and that's ok! You will find divers onboard with all different levels of experience, and some will be suiting up quickly and moving pretty fast to get off the boat and into the water - Let them go at their pace and you go at yours! Even if you feel you are slowing people down, make a reasonable effort to not inconvenience other divers, but you need to be comfortable and do your thing at your pace. There's no shame in taking your time and doing things right!
Most importantly, Have Fun! Diving can be a lot of work, so take your time, savor the beautiful ocean experience in the Keys and make the best of it, and keep it low stress, that way you will be relaxed and be comfortable when you see the amazing things you will see underwater!
Best of luck to you!