Oh, okay. First of all, in case I get some of this wrong, there's an informative site here. But basically, the Merchant Marine is a civilian auxilliary of the U.S. Navy. Apparently--and I may be wrong about this, as I couldn't somehow nail this down--all U.S. flagged ships are part of the U.S. Merchant Marine, and in the event of war, become supply ships for U.S. efforts. But in the meantime, they ply their peaceful way carrying imports and exports in U.S. navigable waters. A member of the Merchant Marine is not called a merchant or a marine, but a mariner. According to Wikipedia, there were 465 ships in the Merchant Marine in 2006, and 69,000 members.
I still don't quite get whether everyone who is considered crew of these U.S. flagged ships is automatically a mariner, or just how all this stuff gets decided. And why do a portion of U.S. ships end up flying under different colors?
However, researching this post reminded me that I did know a bit about the Merchant Marine after all. The Merchant Marine is not one of the uniformed services. Who cares about that, right? Well, apparently, perception is everything. Because even though the Merchant Marine served valiently in World War II, braving many of the same high sea dangers that other branches of the military did, it wasn't until 1988 that President Ronald Reagan signed a bill into law that granted veteran status to those mariners who had served in war. Up until that point, they did not receive any of the benefits granted to other branches of the military.
It turns out that I already knew this. It was part of the story that was told in the excellent documentary The Men Who Sailed the Liberty Ships. I have to admit that I didn't connect this story to the merchant marine, but that's just me. You can check out an excerpt or two here, but do watch the whole thing if you get a chance.