MARDET is smaller than you think, and with some topside covering fire, they will have to wait to engage the enemy.
As for the size of the ship...that does not directly impact getting access to the magazine. Up the forward brow, forward along the deck, enter the ship, down a few decks...
A very long shot, IMHO.
About as long a shot as flying some airliners into the WTC and the Pentagon.
As much as I’d love to see the battleships return (I toured the New Jersey during it’s refitting in the 60’s and tried to do the same in Long Beach but bugged out when I found out I was about 200,000th in line), I’m afraid Poohbah’s view rules the day. The fact that the ships are so old that no one can operate them, as a previous post pointed out, and the difficulty in getting through the armor plate to make changes really rules the whole thing out. Furthermore from a morale standpoint I’m not sure we’d really like the only opportunity to view the Missouri to be from the porthole of a submersible on a National Geographic special.
On the other hand, although we don’t have the infrastructure to make all the thick armor plate, as Poohbah has pointed out many times, I wonder if some updated version of the battleship might not be an option. Something with a very hardened hull, and simple, cheap guns (railguns?) that would not require as many sophisticated systems but deliver more firepower to the target.