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: When we see Jason lying on the bottom of the lake, we also see some chains lying around. At first we see the upper chain lying out on the edge of a plank. But when Jason gets electricity through him we see the chain has moved in to the middle of the plank. (00:06:15)
Factual error: When Jason pushes Julius overboard, he would have drowned within minutes. No one can stay underwater that long without drowning.
Continuity mistake: In the film Sean and Rennie are on board the Lazarus. Sean puts both of his hands into his jacket pocket to get Rennie's gift. He never hands it to her, but In the next shot she has it in her hands. (00:16:55)
Revealing mistake: When Jason throws Wayne into the electrical switchboard, Wayne's jeans catches fire before he even hits the board. (00:49:30)
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