Hi,
To start with, you'll want to be able to refer to the
deck plans:
Titanic Deckplans : Tank Top , especially "Profile" and from E Deck downwards. All watertight bulkheads (WTBs) are marked "WTB 'A'" and so on at the bottom of the plans, and watertight doors (WTDs) are marked with two triangles and the code "WTD".
It might help to understand that there were two types of watertight doors. The ones in the boiler and engine rooms could be closed from the bridge as well as at the door itself. On the other decks the doors had to be closed manually, either at the door or sometimes on the deck above. Once these doors were closed, however, the bulkhead (i.e. vertical wall) was just as watertight as the doors between boiler rooms. If they were not closed, of course, water could flow easily in.
It seems like your question is about E, F and G decks, so I'll focus on those.
On G Deck, there were no openings in the watertight bulkheads. That meant there was no way to walk between compartments on G; you had to go to a higher deck.
On F Deck, most (but not all) of the watertight bulkheads had WTDs in them. These doors, as mentioned, had to be closed manually using a huge wrench (spanner).
On E Deck, we only encounter watertight bulkheads A, B and K through P. That is, the two forwardmost bulkheads plus all those abaft (behind) the boiler rooms. The other bulkheads only reached as high as the bottom of E Deck; A, B and K through P went to the bottom of D Deck (meaning the highest walls were on E Deck). Bulkheads A and B did not have any openings, which was in part because a collision to the bow was considered the most likely accident. K through P had openings secured by watertight doors, again the manual kind as found on F Deck. Between bulkhead B and K, there was nothing to stop the water flooding through E Deck. So E Deck is probably where a lot of the spilling over the tops of the compartments happened. As you can see from the plan of E Deck, continuing the bulkheads up higher in the amidships sections would have meant WTDs dividing up the first and second class accommodations as well as Scotland Road. From White Star's perspective it appears that WTDs in First Class were not acceptable.
None of the decks were watertight. In some places there were compartments with absolutely no hatches, ladders or companionways (stairs), such as in the stern over the shaft tunnels. But in virtually every compartment, for obvious reasons, the floors and ceilings had openings and so were not watertight. So, water had the ability to flow "upwards", as it did when the damaged compartments were filled, or "downwards", as it did when the first five watertight compartments overflowed into the sixth and so on.
I hope that helps some; if you have any more specific questions I (or someone else!) can try to address them.
