But there are sergeants in the Marine Corps, which falls under the "Dept. of the Navy." And, apparently, this particular installation used Marine Corps DI's to train Naval Officer Candidates. Ever notice that there are no USMC "Medical" personnel at the Recruit depots? But there are Navy Corpsmen. No USMC Chaplains, but there are Navy Chaplains.
Sometimes, different tasks are done by different branches of service. And I, for one, am glad they have USMC GySgt's doing OCS training. Because the one question any qualified DI will ask is, "Could I ever salute this person?" So you know they aren't going to turn out anything but a truly qualified individual.
Well, it used to be that way!