IMHO your best bet is to concentrate on the old "inland passage". This corresponds roughly to the modern intracoastal waterway. You are probably not going to find a Spanish galleon there, of course, but don't underestimate what you might discover. The inland passageway was used by Native Americans, the Spanish, English and French. And it was used early by Europeans...from the late 16th century onwards. Many people do not realize the original capital of "La Florida" was the town of Santa Elena in what is now South Carolina.
In antebellum times, the inland passageway was very busy---it was the primary mode of transportation when travelling up and down the coast and was also used to transport the indigo, rice and cotton that was the life blood of the area's economy. There have been many fantastic finds around Charleston, especially in the Ashley and Cooper River areas.
Remember, in South Carolina, lands below the mean high water mark are owned by the state and are held in public trust. There are a couple of exceptions to this---but generally it is perfectly legal to detect anywhere in the state below the mean high water mark---and since the tidal range in South Carolina is so great due to the shallow coastal shelf, that makes for lots of places to detect at low tide. Get yourself a good, waterproof detector and a kayak or other small boat and you are in business. A little research will also increase your chances dramatically, though anywhere along the old inland passage is liable to be productive. Erosion is your friend in this regard. Boat wakes alone do a nice job in some areas of bringing items to the surface or eroding them out of bluffs. Remember, wave action tends to classify material---if a bank is pluff mud, it is probably not going to be a profitable area to detect. But if an area is hard sand---coarser grains---then that area is probably subject to wave action and might be a good place to look. Remember, too, that in coastal South Carolina there is basically no natural rock. If you see rocks or bricks or what looks like "cobbles" along a river bank, get thee excited. Sometimes even what appears to be modern cut granite for seawalls may, in fact, be something else entirely.