39 U.S.C. § 5601
The Postal Service may maintain sea post service on ocean vessels conveying mail to and from the United States.
(Pub. L. 91–375, , 84 Stat. 773.)
Section effective , pursuant to Resolution No. 71–9 of the Board of Governors. See section 15(a) of Pub. L. 91–375, set out as a note preceding section 101 of this title.