When you get to the island, go down to the pier at Harbourtown and/or Shelter Cove Marina. There are several fishing charters at each location.
They all have their sail times and rates posted on boards.
I haven't found a good website for fishing charters yet. Most seem to get out of date or disappear after a while.
[url]http://www.scfishingreport.com/Pages/bodies_of_water/saltwater/hilton_head/hilton_head.html[/url] might be helpful. Check their forums for local fishing discussions.