The state conveys to the United States an easement in an area three square miles or larger, or of different form, in the Nueces County Navigation District, in Nueces Bay, Nueces County, Texas, as designated by the United States, to erect and maintain a fort, military station or camp, magazine, arsenal, dockyard, barracks, lighthouse, naval yard, naval base, naval air base, naval air station, channel, approach for battleships, or any other necessary military purpose.