Continuity mistake: The shards of glass around the porthole change between shots, as Jason attacks Rennie. (00:53:40)
Continuity mistake: When Jason grabs the policeman, the policeman's hat falls to the floor. In the next shot it's back on his head. (01:11:50)
Visible crew/equipment: When everyone is getting off the ship and into the small boat, it's raining. Jason is looking down at them - right when he is looking down you can see above him the sprinkler making the rain. (00:57:40)
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)
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