Attached is the latest SC "Ride" Guide...this was out together by the local kitesurfing community but also applies to windsurfing. Sullivans Island is in there.
"beware of the tidal current"
How can I find out more about this? The first time I got on the water in Charleston Harbour, in a small sailboat, I learned a lot about tides and river currents as the wind died! ( I learned to sail in the Southern Caribbean where we had about a 2 inch tide and zero current.
Tide tables from the almanac are the old fashioned method, I'm sure there are many internet options, probably apps. I prefer marine forecast data, SURFCHEX is a good one. Talk to the locals on the beach, there isn't anything better than local knowledge, if they are all on the water pay attention, if they start bailing, you should to. If none of these options apply it's best to avoid inlets.