You are in the same position my wife and I were when we started diving. The Keys have some of the best dives for newbies, IMHO. April should be pretty good for weather, but you never know. Most dives are quite shallow <35 ft. Check with the Dive Op to be certain that you will be going to those. Most Dive Ops take you out, give a briefing and tell you to be back in an hour or 500 lbs of air, whichever comes first. You are typically on your own, as far as navigation is concerned (unless you hire someone to go with you). Don't worry too much about that. Just do your best. The worst that is likely to happen is that you have to surface to find your boat. Not a problem because of the shallow diving. My wife and I were worried about the initial dives on our own, but everything went well including our navigation. In all likelihood, you will do just as well and have a great time. If you are truly concerned, remember that you can just go out from the boat and back, radiating around it like spokes on a wheel. We came to feel that the shallow Keys dives done without a DM were real confidence builders which propelled us on to bigger adventures.