46 U.S.C. § 55302 — Transportation of United States Government personnel

(a) In General.—An officer or employee of the United States Government traveling by sea on official business overseas or to or from a territory or possession of the United States shall travel and transport personal effects on a vessel documented under the laws of the United Sates if such a vessel is available, unless the necessity of the mission requires the use of a foreign vessel. (b) Regulations.—The Administrator of General Services shall prescribe regulations under which agencies may not pay for or reimburse an officer or employee for travel or transportation expenses incurred on a foreign vessel in the absence of satisfactory proof of the necessity of using the vessel.