Sec. 1. The School Land Board, created by House Bill No. 9 of the Forty-sixth Legislature (being Title: Public Lands, Chapter 3, of the General Laws of the Forty-sixth Legislature, 1939,) is hereby authorized to grant and issue easements or surface leases to the United States of America in accordance with the conditions hereinafter set out, on any island, salt water lake, bay, inlet, or marsh within tidewater limits, and that portion of the Gulf of Mexico within the jurisdiction of the State of Texas, to be used exclusively for any purpose essential to the National Defense.