SC diving tends to be flat, sandy and not always the best visibility. I've seen close to 100' once but 60' is more typical. As you move closer to shore it gets worse as the sand from the beaches is a factor. Deeper out near the gulfstream it gets better but you won't be going that deep as a new diver - those dives often start at 100' or more. The photos on the Express Watersports site are pretty accurate - except maybe too green - I remember everything as more brown due to the sediments in the water. But we were there just after a storm also.
There are ledges there - ledges are hard to describe - they're not really coral reefs but can have some on them. Check out the 1st or 7th pictures on Express's website to see examples. The vast majority of dives they do are wreck dives. Either naturally occurring or they've been put down. Some were small ships, barges, there's even some NYC subway cars at one site.
The wrecks are magnets for fish life. I don't think we saw much of anything once we moved 40-50' from one. Lots of schooling fish and occasional predators lurking near them. No sharks but we saw a few cuda. In summer it will be really warm - like 84-85 but you should still get at least a dive skin as there's jellyfish. To clear them, purge your regulator below them on ascent - we always seemed to run into them closer to the surface. It's not a huge problem though.
If you're with non-divers there's several nice waterfront restaurants in Murrells Inlet also. We stayed in Litchfield Beach in the golf villas area - pretty nice. Maybe 20mins to the heart of Myrtle Beach.
