Merchant Marines were/are basically crew members of commercial ships that worked risking life & limb to fend off pirates or enemies of their home nation and deliver supplies.
Trained like a National Guard or Army Reserve- Merchant Marines get called up during war.
Most famous action of trained & untrained UK Merchant Marines would probably be the evacuation of Dunkirk in 1940.
eta: did a google search, here ya go:
http://www.usmm.org/
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