Continuity mistake: When Jason is holding Eva in the air, he lifts her up higher towards the ceiling, but in the shots of her feet, they stay in the same position.
Factual error: In Manhattan, you can't just climb into a sewer tunnel. They are barricaded, meaning the only people who are allowed down there are city workers.
Continuity mistake: Crystal Lake goes from a lake surrounded by land in previous installments to suddenly having the space for a small yacht and a channel that feeds right to the open ocean.
Factual error: Jason put Shawn's dad up against the ships wheel. Dead bodies can't stand up.
Factual error: When Jason pushes Julius overboard, he would have drowned within minutes. No one can stay underwater that long without drowning.
Chosen answer: To the best of my knowledge, this is just a bit of a continuity mistake. While previous films have not indicated Crystal Lake is connected to the ocean, this film portrays it as being connected. You just kinda gotta go with it for the movie to work.
TedStixon