The watertight bulkheads were the watertight walls of the Titanic, they were used to hold sea water and keep the ship afloat if case there were any damaged compartments on the ship. There were 16 of them, and the ship could stay afloat if 3 of them were flooded, or the forward 4, but if 5 or more were flooded, the ship wouldn't be able to float, and would instead sink. Most of them extended up to the E Deck and several up to D-Deck, not high enough to prevent her from foundering on that fateful night.
Sinking[]
During the sinking, the watertight bulkheads sadly weren't tall enough to hold the water spilling in from the damage, and water ended up flooding from the lower decks: Orlop Deck, G Deck, F Deck, E Deck, to the higher decks: D Deck, C Deck, B Deck, A Deck, Boat Deck. The weight of the water would pull her down by the head, and then the compartments were filled completely and went on to overflow into the next.
After the Sinking[]
The watertight bulkheads on Britannic and Olympic were heavily modified so in case they had a similar form of damage like their sunken sister, they would stay afloat, since the bulkheads would be extended all the way up to B-Deck. They received a double hull as well.
Sadly, it was proven once more that no ship is truly unsinkable and even the best of measures cannot always prevent the biggest disasters from happening. During World War 1, on May 7th 1915, the Lusitania of White Star Line's rival company Cunard Line was just 11 miles off the coast of the Old Kinshead, Ireland, sailing to Queenstown was attacked by the German submarine U-20, and sank after only one torpedo had struck her. The impact caused a huge second explosion as well as a massive hole in her hull, and she listed heavily to starboard side, and went down in only 18 minutes. About 1198 people on board where killed. Probably there was munition aboard. But the bulkheads couldn't keep the rushing water back.
At the Aegean Sea in the morning of November 21st 1916, also close to shore, where she served as hospital ship, Britannic had hit a mine that was laid by a German submarine U-73, and the explosion caused the hull to warp a bit, enough to jam the watertight door. On top of that, several portholes were opened. She went down by the head and at the last moment, she rolled completely on her starboard side, and only sank in 55 minutes, much less time than it took Titanic to sink despite all her improvements following the Titanic disaster.