A single anchor means the sailor crossed the Atlantic or has been a member of the merchant marine, a fleet of civilian ships that carries military cargo. In wartime, this fleet is mobilized to carry war materiel, including troops and supplies.
During World War II, the Merchant Marine took a beating with high casualties, entering the European war long before the United States itself. Since the U.S. was delivering war supplies to Britain through Lend-Lease, Nazi u-boats targeted U.S. shipping bound for the UK. The Merchant Marine casualty rate was 3.9 percent, whereas the Marine Corps’, the next highest, was only 2.94 percent.