Good luck, I am thinking about that area this spring as well.

We stay at Hunington Beach State Park, just up the road a piece from GT, and fish Murrells Inlet, we did pretty well last year around Easter.
I was talking to a guy there at the campground and he fished the back creeks just above GT, and tore up the Reds and Trout, and had a couple of nice Flounder in the cooler to boot.

We flush at the campsite, but some of the public ramps have fresh water hose bibs to wash down with before you leave. But if they are anything like the ramp at M.I. it gets pretty busy.

Here's a website I found that list the public/private ramps in Georgetown County : http://saltwaterfishing.sc.gov/boatlandings.html I need to locate the two ramps this guy used to fish from, so while we are there this spring I can try it.

Worse case is one of the private ramps would have freshwater, and I am sure for a fee they would let you use it.

Let me know how you do.
Kevin