Mercantile Marine Medals / Merchant Navy Medals World War One 1914 to 1918. The main medal for British Merchant Navy personnel in World War I was the Mercantile Marine War Medal, awarded for service at sea for at least six months or for one or more voyages through a danger zone between 1914 and 1918. Recipients of this bronze medal were also entitled to the British War Medal. Both medals were issued by the Board of Trade