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 must travel and transport personal effects on a vessel^* documented under the laws of the United States 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 must prescribe regulations under which agencies cannot 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.