Oh, looking over the deck plans... I didn't include the many small stairways that go from ash ejector rooms on F Deck to "ash places" on E Deck. Like I said, the objective was to figure out how one could get to the Boat Deck, and those stairways are all dead ends.
Ajmal Dar: Titanic didn't have any watertight decks, with the exception of a few spaces such as the shaft tunnels where there were more or less watertight ceilings on some deck levels. I can't find the exact testimony but one witness at one of the inquiries (I suspect Wilding at the limitation of liability hearings) testified about watertight decks. He said that such decks could be a liability because air space trapped below flooded spaces can cause serious problems with stability. That is, you have a compartment full of air trapped below the waterline, and the buoyancy of that compartment can cause the ship to capsize. The same is true when ships have a longitudinal bulkhead, as Lusitania and Mauretania did. Longitudinal bulkheads divide the ship lengthwise; if Titanic had one, then the starboard side forward compartments would have flooded but the port side would not. This may sound like a good thing, but in fact it makes the ship liable to capsize. Wilding calculated that Titanic would have capsized quickly (perhaps within a half hour) if it had had longitudinal bulkheads. As you perhaps know, when the Lusitania sank it capsized quickly, and the longitudinal bulkheads were the main reason.
As you said, the ship would have great difficulty functioning with watertight decks. To avoid the situation I just discussed it is necessary to have the option of flooding compartments that are undamaged. It is also necessary to have a lot of hatchways and watertight doors to protect stairways/ladders. On warships (in my limited experience with museum ships) you don't have stairways, only ladders with hatches on top.
