The UK poster of the ship underwater isn't actually the Poseidon. It's actually the P&O Ferry lines Arcadia II. This is because they hadn't designed the ship when the poster was released. See more...
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Factual error: Toward the end, when they are in the bow thruster room, the ship's seams rupture and water, under high pressure, streams in. They had to drop 30 ft from the bow thruster tube, that room would have been at, or just above the water level. 1) The seams wouldn't have burst like they did, and 2) the water wouldn't have streamed in at such high pressure.
Continuity: During the capsize, when the woman falls out of the elevator to her death, she is flipping a different way when she impacts the roof from when she first started falling. Also, during this same scene, when we first see the elevator coming loose, we can see that the person inside is a man in a tuxedo. But, when the person falls, it is a woman in a cream-white dress.