In Marathon try Tilden's SCUBA Center phone 305-743-7255 they are at mile marker 49.5 (if you have never been to the keys every places location is identified by mile marker) they will boat you out to Sombrero. Also, if you don't have a place set up to do your referral they will do this also. Other dive shops in the area: Capt. Hooks mm 53.0, 305-743-2444, C.J.' Dive Center mm 48.5, 305-289-9433, The Diving Site mm 53.5, 305-289-1021. Looe Key is Government Owned and operated I can't find their number but, I did find the Marathon Chamber of Commerce phone number - 305-743-5417 (they are open 8:30 to 5 mon-fri - would be a great place to visit 1st when you are down here mm 53.5 see what's going on in the area) give them a call - they may have a sight on the internet, and see if they can set you up on Looe Key dives.
Anyway, about diving in general - it's normal to be a little apprehensive prior to your OW dives, this is good, gets the blood flowing. Just remember, it's just like in the pool only a little deeper. These first few dives that you make will be the safest you'll make. Lots of supervision by the Instructor. The hardest portion will be doing your "skills" mask clearing, regulator recovery, etc. Once you are done with these the rest is easy -- it's just diving and diving IS FUN. One of my sons who has been diving with me for years always tells people that the certifing agencies always make getting certified so hard. Once you have your "C" card "diving is easy". We do make certification a little hard because we want to put good, safe, confident divers in the water but, my son is right - diving is easy. Now just go do it and have a great time. I want a full report once you're back home!