Factual error: The amount of people on the ship is very unrealistic. Although the low number of people is to benefit the plot, there should have been cooks, waiters, deckhands, boiler room engineers, cleaners etc. (00:40:00)
Other mistake: Rennie shouldn't have been able to get on the boat at all. Being high school students, they would need a permission slip signed by their legal guardian to board the ship, just like any field trip. And, it's obvious that Charles would not sign one allowing Rennie to board. He definitely voiced his objections to the female teacher. Rennie can't make that decision on her own as she did.