Someone has already mentioned the three stripe/four stripe epaulettes worn by the First Officer. Three is correct. In some scenes both are 3 and others, both are 4. But in the final scene, he manages to wear 3 on his right shoulder and four on his left.
When the captain is called down to the pump room after the lights go out, the voice on the walkie-talkie is of a character who was already murdered in front of the group. The captions even put her name.
In the bar when Terry is trying to expose the killer the number of rings on Justin's right uniform epaulette alternate between three and four depending on the camera angle.
The ship seems to have only one man working in the engine room. When he dies, the Captain and First Officer take over. In reality, there are officers (engineers) and crew in an engine room, especially on a big ship with passengers.