Most of the diving is off of artificial reefs, closer inshore dives usually at 3 mile barge, or some bridge rubble(or stacks-n-pillars). Offshore dives in the 10-20 mile range are plentiful with tugs, barges, planes, army tanks, even large liberty ships(the Wallace being 400' long). Go out a bit farther and you can hit some of the rigs. There are some limestone reef/ledges, but most are deep(100-130') and 20-30 miles out. Some of the best Red Snapper fishing available out there(out of GS and OB)
grouper and amberjack are common place as well as smaller fish(sheepshead, flounder, ect).
If you like diving on wrecks, its a great area IMO. I'm not sure how many are open enough for penetration, so I'll let someone else comment on that.
I've done several dives out there on several sites including 3mile barge(inshore), Navy tug, 22 mile tug, Corsair, Wallace, ect. Had fun each time, came home with flounder each time I've been out there with my polespear. I looking forward to breaking in my new speargun out there as well.