Revealing mistake: In the scene when the crazy guy is hit in the back with the axe, when he falls, the axe sways side to side a little bit, then stops a couple inches of center from before.
Revealing mistake: When Jason breaks his hand through the door it's really strange how his hand makes a perfect square on the broken door. Telling that the door was already broken and then repaired loosely for the stunt.
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