[Warning: Spoilers dead ahead.]
⊕ The vessel has an important mission to accomplish.
⊕ Someone on board has a hidden agenda.
⊕ Someone on board is mentally unstable.
⊕ The captain has a drinking problem.
⊕ The hull of the boat/ship/sub will spring a leak.
⊕ The motor of the bail-out pump will overheat and explode.
⊕ A klaxon horn will sound and emergency lights will come on.
⊕ The vessel will lurch and items in the galley will crash to the floor.
⊕ The mentally-unstable person will secretly perform an act of sabotage.
⊕ A steam valve will burst, badly burning a non-essential crewman.
⊕ The engineer will be trapped by rising waters while trying to restart the engines.
⊕ A vital cable and/or air hose will break, causing a non-essential crewman to drown.
⊕ The captain and first mate will have a private discussion about the deteriorating morale of the crew. They will drink coffee from tin cups and one will spit his coffee onto the floor.
⊕ The navigation and/or communications systems will fail.
⊕ Someone will rap his fingers on the glass of a gauge, disbelieving its accuracy.
⊕ A young crew member will come up with a makeshift solution to a temporary crisis by using an item of obsolete and/or defective gear.
⊕ The captain will be unable to control his drinking and will retire to his quarters in a delirious sweat, forcing the first mate to take command.
⊕ The person with the hidden agenda will take advantage of the chaos on board to advance his petty plan.
⊕ A blind radio operator on his last assignment will help the first mate navigate through a minefield and/or icebergs and/or underwater mountain range by placing his ear up to the hull and tapping on it with a wrench.
⊕ Meanwhile, the makeshift repair threatens to fail at any minute.
⊕ The damaged vessel will return to port. The dead include the engineer, the saboteur and two non-essential crewmen. Only the engineer will be mourned.
⊕ The captain will be carried off on a stretcher. The person with the hidden agenda will be taken into custody.
⊕ The film will close with the first mate standing at the helm of his own boat/ship/sub. It will be a sunny day and there will be a fresh sea spray. The end.