S.C. Code Ann. § 4-3-5

For purposes of describing the boundaries of counties when a county boundary is used to establish or describe the boundary of a jury area in Chapter 2 of Title 22, the center line of waterways is defined as: (1) for areas influenced by the tide, the center line is the line described by a series of points equidistant from the banks as delineated at mean high tide, except when an island is present; then the center line follows the main channel around the island and is the line described by a series of points equidistant from the banks of the main channel as delineated at mean high tide; and (2) for areas not influenced by the tide, the center line is the line described by a series of points equidistant from the banks as delineated by the ordinary high-water mark, except when an island is present; then the center line follows the main channel around the island and is the line described by a series of points equidistant from the banks of the main channel as delineated by the ordinary high-water mark.