merchant marine       
noun [ C or S ] US uk/ˌmɜː.tʃənt məˈriːn/ us/ˌmɝː.tʃənt məˈriːn/ (UK merchant navy)
 商船船队
a country's ships that carry goods and are not used for military purposes
  The merchant marines suffered a higher casualty rate than any branch of the armed services. 商船遭受的人员损失比任何兵种都大。 
 He served in the Merchant Marine.  他在商船上工作。