Factual error: When the ballast tank valves open, they open by dropping down, which would be impossible with the pressure of the water pressing against it. Regardless of the domino effect of the water flowing, it's tremendous pressure.

Factual error: After the first ballast tank is flooded, the others are shown filling during the rest of the movie. But since the ship is upside down and the valves therefore at the top of the tank, all of the water is below the valve, and couldn't possibly flow through it to the next tank.

Plot hole: When the ship has turned on its side for the brief moment, it cuts to inside the ship where a hallway explodes, causing the ship to submerge. The only problem is the people are standing on the ceiling, but if the ship's on its side, they'd be on the wall, not ceiling.

Audio problem: The source music in the ballroom does not match the Poseidon orchestra's motions.