Part of the revolutionary design of Titanic was her sixteen watertight compartments, which were aligned front to back along the ship's length. Any two of the compartments could withstand flooding or four of the first five forward (bow) compartments could flood, and the ship would stay afloat. But, as you probably remember Thomas Andrews saying in the film -- not five, which is, unbelievably, exactly what happened; the iceberg opened the first FIVE compartments to the sea.
Another problem was that the bulkheads between compartments 5 and 6 only extended as high as E deck -- in other words, there was enough space at the top for water to spill over. As the bow was pulled down by the weight of incoming water, it flowed over into each successive watertight compartment -- like water in an ice cube tray.
Shipbuilders heading home from the Harland & Wolff yards at the end of a long day. You can see Titanic in the background.
Titanic's outfitting berth at Thompson's dry dock. Queen's Island, Belfast, Ireland.