16 U.S.C. § 459f
For the purpose of protecting and developing Assateague Island in the States of Maryland and Virginia and certain adjacent waters and small marsh islands for public outdoor recreation use and enjoyment, the Assateague Island National Seashore (hereinafter referred to as the "seashore") shall be established and administered in accordance with the provisions of sections 459f to 459f-11 of this title. The seashore shall comprise the area within Assateague Island and the small marsh islands adjacent thereto, together with the adjacent water areas not more than one-half mile beyond the mean high waterline of the land portions as generally depicted on a map identified as "Proposed Assateague Island National Seashore, Boundary Map, NS-AI-7100A, November, 1964", which map shall be on file and available for public inspection in the offices of the Department of the Interior.
16 U.S.C. § 459f