One of my uncles was a MM in WW2. Murmansk run, for awhile. There was a lot of positive press about MM in WW2. They were highly lauded as part of the war effort. But what was not publicized was this: often when a ship came back to places like NYC or Bayonne MPs were at the dock. The voyages were so dangerous that large parts of the crew would desert rather than go out again, or at least that is how the Brass saw it. The MPs were there to make sure no one got off the ship, except officers.
As the war went on, and the U-boat menace declined, he said, they were later allowed to go ashore more or less freely.