Well, it's a fiction prompt. In a fiction story and they should be allowed. The question you should ask yourself is what's the reason for zombies or aliens to invade a ship that sinks the same or the next day.

The ships I've included in the 3 options are all sinking soon after they sail off their ports. You should somehow describe or include the shipwreck event in the story. This would give you an advantage when the jury is reading and rating your submission.

Hope this helps! :)