The United States Maritime Service is not synonymous with the United States merchant marine. Rather the Maritime Service was responsible for training officers and sailors for service in the merchant marine. The Maritime Service was especially significant during World War II when it was necessary to train tens of thousands of men in short order to meet wartime needs.
The Maritime Service still exists today, in greatly reduced form, in that instructors and administrators at the U.S. Merchant Marine Academy and the six state-run merchant marine colleges often hold commissions as officers in the Maritime Service.