I did my tech training at Horizon Divers with Jeff and I would highly recommend diving with them. I just got back from a Key Largo trip with them recently and in addition to some of the great tips already on this thread my $0.02.
Planning ahead as a single diver is key. If you let them know ahead of time you're single they can arrange a guide or put you with a guided group if there is one. They may also be able to tell you if there is another single diver you can try to buddy with. What is suboptimal is just showing up on the boat with no buddy and no plan and expecting the crew to sort it out for you. It's a friendly family run dive shop and most of the divers are a mix of tourists, regulars, locals etc. so if you dive with a guide/instructor the first day or two, when you get on the boat, you can make some new friends and find dive buddies for the rest of the trip.
All the rec and tech DMs and instructors are top notch. Experienced and competent. Really worth it if you want to tour inside the Spiegel Grove. There is a lot to see in the 70 ft/21 m - 100 ft/30 m depths and the bottom is at 130 ft/40 m. The super structure has been cut out and can be safely penetrated by competent rec divers. I have 15 dives on the SG and I just used a guide and will continue to use guides to see the coolest bits of the wreck. Here's another thing, not all dive shop (Rainbow Reef?) guides are competent inside a wreck. If another dive shop takes a large group of tourist divers through the passages, viz will be ****. So it's nice to have a guide who is familiar with all the alternative less traveled paths and photo ops. Try to schedule a guide in advance.
Jared drew the maps. IMHO his map slates are worth buying to plan where to see and much clearer and more detailed than the ReefSmartGuides maps.
Highly recommend taking a course. Unlike most of the other dive shops Horizon offers AM and PM deep dives. It's possible to dive the SG 4x rec dives or 2x tech dives in one day. e.g. AOW, Nitrox, Deep, Solo, Wreck, technical etc..