Any enclosed space could crumble pretty quickly due to the fact that no matter how a person dies (except head trama) they come back as a zombie; especially at the start of the issue when not everyone knew that. One of the astronauts on the ISS knows that his whole family has died and commits suicide then attacks the rest. Closing off compartments on a submarine full of zombies with also be problematical. Still, if it has been only a few months at the start of the show, the military would still be up and running pretty well. Tanks are fast enough nowdays to just plow into herds of zombies and just mow them down, enter back into a fenced area, close the fence, marines kill off the zombies that made it in, refuel the tank, rinse and repeat.
Still, it is their show. A show about zombies. Some suspension of belief comes with the territory.
I predict Daryl coming back rather quickly and finally consummating his relationship with Carol.