There really isn't much difference. Obviously, a Great Lakes seaman is an individual who sailed on the Great Lakes. However, in order to sail on the Great lakes or on any vessel in the U.S. Merchant Marine, an individual must have a Merchant Mariners Document issued by the U.S. Coast Guard.
I don't know if this is one of the individuals you were asking about but my records show a Charles Popham sailed as Chief Cook on the W. E. Fitzgerald, operated by Gartland, in 1948. The W. E. Fitzgerald was launched 9/8/1906 at Wyandotte, MI. She was 440 x 52 x 28 and had a rated speed of 13.8 mph. On 12/14/26 she damaged hull frames and plates while crossing Lake Superior. She was converted to a self unloader in 1928 and sold to Gartland Steamship in 1932. The W. E. Fitzgerald was laid up for the last time in 1968 and scrapped at Humberstone, Ontario in 1971. Gartland Steamship went out of business in the late 1960's.