Trish: Here's an excerpt from a website I found searching for "Navy Cross":
"The United States, with the Medal of Honor as its sole decoration, was caught unprepared not only for this custom [decorating soldiers for heroism in international wars], but also had no appropriate award to recognize heroism of a level less than that which would merit the Medal of Honor and no decoration to reward the myriad acts of meritorious non-combat service that the war would spur. The U.S. Army shared this dilemma and with the aid of President Woodrow Wilson and the Congress in early and mid-1918 instituted its Distinguished Service Cross and Distinguished Service Medal (DSM) with clear guidelines for the award of the Distinguished Service Cross for combat heroism and the DSM award for distinguished non-combat duty in a position of great responsibility. This pair was available in time for awarding during World War I. Parallel awards were created a year later [the Navy Cross] for the Navy and Marine Corps, months after the armistice and amid the massive demobilization of our forces."
Marines are part of the Navy, and, among other duties, provide security on warships.