Jordyn, I’m no submariner, but could it be the main pressure vessel hull? Most of the visible hull is built around the actual hull, which is approximately cylindrical in shape. See the below link, or construction pictures of any of this class boat, on Navsource (via Google).
Also if you ever get to do some reading, there are many harrowing stories about the silent service in WWII. It was a vital part of Naval strategy and reading up will give you a fine appreciation for the boats and the men who sail them.
Navsource link to the Wahoo (a boat with a storied history). Note the first few pictures of construction.
I suppose if you wanted to do a cutaway model, some find it interesting to detail the innards of the vessel, hence the semblance of bulkheads, if I’m not mistaken.