USMS stands for United States Maritime Services
"The Secretary may establish and maintain a voluntary organization for the training of citizens of the United States to serve on Merchant Marine vessels of the United States to be known as the United States Maritime Service...and to prescribe the uniform of such service and the rules governing the wearing and furnishing of such uniform....The ranks, grades, and ratings for personnel of the United States Maritime Service shall be the same as... for the personnel of the United States Coast Guard." Merchant Marine Act of 1936
In 1938, when a second World War was imminent, President Franklin D. Roosevelt realized that winning the war would require many ships to carry war supplies to the fronts. He ordered mass-production of Liberty ships and established the U.S. Maritime Service (USMS) to train the men needed to operate these ships. Joseph P. Kennedy, (father of President John F. Kennedy) was appointed as the first Chairman of the new Federal Maritime Commission in 1937 during which he laid the groundwork for the U.S. Merchant Marine. Kennedy became the United States Ambassador to Great Britain in 1938. Retired Admiral Emory Scott Land, USN, succeeded Kennedy as Chairman, and who also headed the War Shipping Administration. Admiral Land is the genius who put together the ship production, training of the men in the USMS, and operation of the vast fleet
Here's the link if you would like to read more
http://www.usmm.org/usms.html