A time period is important. Have you traced his life back? Would he be in a census with occupation recorded? Since you say he had to get a new Social Security card then his employment history sould be in their records. Here is a link that explains about the merchant marines:
"About the Merchant Marine The U.S. Merchant Marine (USMM) is a fleet of privately-owned containerships, tankers, passenger vessels, tugs and other types of ships engaged in domestic and international commerce. They are regulated by the Department of Transportation's Maritime Administration (MARAD).
The Merchant Marine has ties to the Coast Guard, Navy and other armed forces. But, while they might be veterans, crew members are not active members of the military. However, Merchant Marine officers are also commissioned officers in the Naval Reserve or another branch of the military reserve. During wartime or national emergencies, the military might call on the Merchant Marine to transport troops and military goods.
Merchant Marine Jobs Employment in Merchant Marine jobs is the responsibility of ship owners, but job opportunities are typically through maritime labor unions. Apprenticeships are available, but part-time jobs usually are not."