(a) Proscription by Presidential proclamation
Whenever the President shall have issued a proclamation under the authority of section 441(a) of this title it shall thereafter be unlawful for any citizen of the United States to travel on any vessel of any state named in such proclamation, except in accordance with such rules and regulations as may be prescribed.

(b) Revocation of proclamation; effect
Whenever any proclamation issued under the authority of section 441(a) of this title shall have been revoked with respect to any state the provisions of this section shall thereupon cease to apply with respect to such state, except as to offenses committed prior to such revocation.