Merchant marines — Are they commercial freight-hauling seafarers that are available for military duty during wartime? Are they mercenaries? Are they something like the National Guard or the Reserves (“weekend warriors”)?
In short, they are the officers and sailors that work on boats and ships operating under the U.S. flag, save the armed sevice vessels of the Navy and Coast Guard, although some Navy and CG officers come from MM academies. I work with a few of them.
When you see a freight ship, or a tank ship that is U.S. flagged, the crew is U.S. Merchant Marine. The officers most likely came from one of the MM academies such as Kings Point, Mass Maritime, Maine Maritime, etc, which are similiar to other 4 year service academies.
Some merchant marine sailors work aboard ships of the Miltary Sealift Command.
They are basically the country’s civilian professional mariners.
Nomenclature nitpick:
The merchant marine is the collection of all ships registered and flagged in a country.
The people who sail on them are called merchant mariners, individually or merchant marine collectively.
The word marine and its plural, marines, when used of individuals, is pretty much reserved for web-footed soldiers, (those fighting men, originally carried on ships with no naval duties beyond defense of the ship, who made up landing parties and who, in the U.S. (and to a certain extent, Britain) were organized into larger military forces for amphibious landings and, later, general combat).