My wife felt similarly to you on her first ocean dive (which was also in the Keys). However, once you're down, and you get your first close-up look at the reef, I think you'll find your biggest concern will be that you'll have to return to the boat before you've seen everything there is to see.
A guide is a good idea your first time out, more to give you a guided tour, and comfort level, than to make sure you don't get into any trouble. I have seen *many* OW students kneeling on the sandy bottom, next to Molasses, or French reef, or in site of the Benwood, going through their skills... All, easy, fascinating, beautiful dives...
Enjoy yourself... (although, I have no doubt you will...)
A guide is a good idea your first time out, more to give you a guided tour, and comfort level, than to make sure you don't get into any trouble. I have seen *many* OW students kneeling on the sandy bottom, next to Molasses, or French reef, or in site of the Benwood, going through their skills... All, easy, fascinating, beautiful dives...
Enjoy yourself... (although, I have no doubt you will...)