Continuity mistake: When Rennie gets up and walks over to the closet, you can see some items on the side, such as a decorated silk cloth, a bag etc. When Jason then looks through the porthole into Rennie's room, the position of all the items are different. (00:21:35)
Visible crew/equipment: As Jason is walking down some steps, a little after he kills J.J., you can see the shadow of a crewmember pushing equipment, cast on the wall. (00:21:55)
Visible crew/equipment: When the boxing scene starts, as the camera pans past the punchbag, the shadow of the camera is clearly cast on it. Then as the camera continues to pan past the punchbag, you can see the sides of two crewmembers come into view (who are following the path of the camera.) They are both wearing a jumper and jeans. (00:23:05)
Continuity mistake: When the two girls are sniffing cocaine their hair changes positions. (00:25:20)
Visible crew/equipment: When Rennie is searching for Toby, the camera then pans to the games room, and reflected in the four pictures hanging up on the wall is a crewmember giving Jason a signal to come on cue. (00:26:00)
Continuity mistake: The position of the hot rocks in the wooden box, and the position of the spoon in the bucket changes through the sauna scene. (00:27:40)
Continuity mistake: The towel on the guy's head in the sauna changes position between the shots. (00:28:15)
Other mistake: At the beginning when you see outside the museum, there's a man in a red T-shirt looking at the camera. (00:28:45)
Continuity mistake: In the film Jason kills the guy in the sauna with a hot coal into his stomach, and there's no blood. When Jason leaves you can see the guys chest and now it's covered with blood. (00:28:45)
Visible crew/equipment: When Tamara pushes Rennie, as Rennie starts to fall you can see a crewmembers hands reach out to catch her. Look close and you can even see the red tee-shirt sleeve of the person. (00:29:20)
Continuity mistake: When Rennie is being pushed overboard, notice the guys sitting in the background wearing a blue shirt. When it cuts from below the ship, as we see Rennie falling, the guy is gone. Also when Miss Van Deusen goes for the life ring, we see from below the ship that the guy is gone. But when it cuts he is there again. (00:29:20)
Revealing mistake: When Tamara pushes Rennie overboard, the ship stays in the exact same place, as if it was docked. In reality, the boat would have sailed on, leaving Rennie behind. (00:29:25)
Revealing mistake: When Tamara pushes Rennie overboard, the water is calm and there are no waves. (00:29:25)
Revealing mistake: As Sean dives into the water to save Rennie, in all the overhead shots Rennie has dry, curly hair, but in closeups her hair is soaked down straight. In the overhead shots it's obviously a stunt double. (00:30:10)
Continuity mistake: The position of the towels on Rennie's shoulders change. (00:31:20)
Continuity mistake: When Rennie raises her hands and notices the blood on them, the position of blood on them changes in the next shot. (00:32:00)
Continuity mistake: When Jason, as a kid, comes out of the mirror and grabs Rennie by her throat, Jason's grip on her throat differ between the shots. (00:32:00)
Continuity mistake: When Rennie goes to the bathroom after the scene where she gets thrown overboard, her hair is still wet but her clothes are completely dry. (00:32:05)
Continuity mistake: In the scene where Tamara gets out of the shower, her silk gown which hangs on the handrail changes position between shots. (00:35:45)
Revealing mistake: When we see a wide shot of the ship in the storm we can see the ship slide through the waves untouched by them. It's obvious the scene is a composite. (00:35:45)
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