S.C. Code Ann. § 5-7-150

Every coastal municipality has criminal jurisdiction over piers and other structures and the waters of the ocean, a sound, or an inlet within one mile of those portions of the strand within the corporate limits. The corporate limits of the municipality are extended in a straight line from the strand into the ocean, inlet, or sound from the point where the corporate limits of the municipality reach the high-water mark of the strand. If an extension overlaps with the criminal jurisdiction of another political subdivision, the jurisdiction of each political subdivision extends to the equidistant point from the high-water mark of each strand.