Continuity mistake: During the capsizing, the piano crushes two passengers against a wall. When the piano falls to the ceiling/floor, you can see the same two people fall, but are very much conscious/alive.
Continuity mistake: At the end of the capsizing, Terry falls from the table into the ceiling light, and there are people standing around, clutching each other, which shouldn't be because they wouldn't have had time to recover. When the emergency lights come on, everybody is strewn and about to recover, nobody is standing clutching each other.
Factual error: Throughout the movie, the characters talk of "the" engine room. The Queen Mary (aka Poseidon) has four engine rooms: forward port, forward starboard, aft port, and aft starboard. There are also four shaft alleys (one for each propeller shaft), along with two generator rooms and five boiler rooms.
Other mistake: There was no way the dining room could have filled up with that much water in that amount of time.
Other mistake: In the scenes bookending the underwater swim, there is no way to figure out how both pools of water are set up. Sometimes a cast member jumps in and appears to be standing in 3 feet of water and other times cast members dive in as though the water is 25 feet deep. In fact, the whole underwater scene makes no sense at all. Cast members swim up meaning they're going through a hatch in the ceiling or floor, there are railings attached willy-nilly and there appears to be no regular doors anywhere.