Chris, I'm down there a couple times every year. One thing about it, the area in general is better for birding than bird photography. You've got the 500/4P which is the same kens I use. My best luck spots are off the causeway going from Chas out to Folley. Nice marsh grass areas that, at falling tide, are nice for smaller shorebirds and the usual waders. You're a late for migration, but there's a healthy summer population of peeps, so if can get close enough, you should have some variety. Of course, this time last year, I got a nice Black-bellied plover in breeding plumage off the causeway - so you never know!
Other productive areas are...
Pitt St Bridge in Mt Pleasant. Tidal flat on one side and salt marsh on the other. Good spot to pick up marsh wrens if you don't mind scrambling down off the old bridge and setting up a flash and being patient while squeaking your head off (they're everywhere, but a bit hard to get up long enough for a shot).
Also, with proper tide timing, the landing and pier at Cape Romain is potentially productive. If you don't mind spending a little time in the car, Huntington Beach State Park a little over an hour north is very good for beach shooting. Piping plover nesting sites and peeps everywhere. If you get in an artsy mood, the old house site in the park (I forget the name, but it was owned by Vanderbilt), is a neat place to get out of he sun and do some neat scapes. Across the highway from the park is Brookgreen Gardens - one of the largest statuaries around. The gardens are legendary, and the ancient live oaks are spectacular.
Too bad I was down there just last week! I'd love to meet up and do some shooting!!