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

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. 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.