Conventional theory in 1912 was, a ship sinks like a bucket. Air is replaced by torrents of water. A ship is big, therefore it pushes water out of it's way as it sinks, therefore, water must rush in to replace it. In essence, this creates a void, vacuum, suction to be filled. It's wrong, but it was the easiest assumption. Most likely, the massive number of deaths in the water were from the frigid temperatures. Drowning? Yes, probably some with out life jackets or those unconscious from striking objects in the water. Water pressure? Most likely if they were still inside the ship, and some had to be.
Here's a thought, though probably ghoulish and only from curiosity- what would exhumation of the unidentified bodies tell a forensic anthropologist? Bones are good witnesses, and they never lie.