I have lived in the area for 10 years and have dove the area for 15 years. I can share with you what I know.
Edisto beach is known for fossils, BUT where they are hiding is a different story. I found references that fossils were found at Jeremy Inlet (about a mile north of Edisto). So I walked up the beach to Jeremy Inlet and found fishermen and billions of shells. I have never seen so many shells. I snooped around for hours and dug up shells and more shells. The shells were literally in layers. Not "pretty" shells. Just your plain oyster shell type variety. I did not find ANY fossilized material what so ever.
Having been a diver for 15 years one develops a sense of what the underwater profile looks like. My estimate of what one would find diving off the Edisto beach is a bare slowly sloping featureless pure sand bottom. A word of caution, we do have large shrimp boats slowly running the coastline so you would not want to get in their nets.
There are some walk-ins in the Edisto River.
Johnny Cercoperly can tell you their locations
I dove a ledge offshore Charleston SC last season and found limestone. Did not take long to see that it was filled with fossils. I dug out a large fossilized scallop. The other divers traversed on past basically because they did not know what they were looking at, or had no interest in fossils. The ledge is named the "Anchor Ledge" and is a dive site visited by "Charleston Scuba". It is on their website on their list of "reefs and ledges"
For offshore Charleston diving I use
Charleston Scuba. There is a newer smaller shop in Mt Pleasant but they focus on spearfishing. Not my cup of tea at the moment.
Next stop for offshore diving for me personally would be 1.25 hour drive north to Murrrels Inlet and
Express Water Sports. Great boat. Good outfit.
ps: There is an infinite number of very rich potential dive site locations in and around Charleston. I imagine many are overrun with civil war artifacts, early American artifacts, and fossils. Most will be found in the inland waterways. Or I should say most will not be found and will lay hidden forever. Most divers, including myself, stay on the well beaten path. We do have old Spanish and French shipwrecks in the waterways. We do have local divers with underwater metal detectors finding old Spanish and French coins. The materials are there. Its just a matter of searching the unknown and finding it.