A couple of years ago we launched at the "WO Thomas Jr. Landing" boat ramp, which is immediately next to the Rivers Edge Marina on the Ashley River. These are located where highway 526 crosses over the Ashley River. It was a good ramp and we kept the boat at the marina for 2 days to get ready for a cruise up the ICW. We saw vehicles and trailers overnight. You could call the marina to double check ramp parking, although I don't think they have any sort of control over the ramp or parking lot. From the ramp/marina it is a quick easy cruise into Charleston Harbor.
Rob