Seaward boundaries of original coastal states extend to three geographical miles from the coastline (or, in the case of the Great Lakes, to the international boundary).

States admitted after the formation of the Union may choose to extend their seaward boundaries to three geographical miles from the coastline (or to the international boundary). [43 U.S.C. §1312] Except under a final decree of the U.S. Supreme Court, no state’s seaward boundary may extend beyond three geographical miles from the coastline into the Atlantic or Pacific Ocean or three marine leagues into the Gulf of Mexico. [43 U.S.C. §1301(b)]