James Cameron is known for his perfection. He has a reputation for doing crazy things to get details right. He used blueprints of the real Titanic to build his set and he did an amazing amount of research for the film. He even went down to the ship and went through it with a camera-equipped robot.
Of course, sometimes you need to make concessions for dramatic effect. For instance, at the end of the film Titanic Jack and Rose are on top of half of the ship bobbing in the water. It stays like that for about 30 seconds and then falls into the water and creates a weird suction kind of thing. Guess what? That never happened. Instead, the second have of the ship was pretty horizontal and never got vertical, it just kinda fell below the surface of the ocean. Whoops!
These photos, provided by the good people of BuzzFeed, show the difference. [BuzzFeed]
National Geographic video: