You are required to travel by U.S. flag ship for the entire trip, unless use of a foreign ship has been authorized by your agency. Any cost that is attributed to improper or unauthorized use of a foreign ship is your responsibility.
Code of Federal Regulations
[FTR Amdt. 70, 63 FR 15955, Apr. 1, 1998; 63 FR 35537, June 30, 1998]