Other mistake: Jason seems to be able to teleport in this film. Most notably towards the end when Mr. McCulloch is running from him down an ally, Jason is behind him, then in a matter of seconds he's on the top floor of a building, without ever entering it, in time to throw McCulloch out the window.
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.