If you pause this film at 53 minutes 56 seconds you will see the error. This is where the killer is chasing Jennifer Morrison's character because she the security tape he wants. At this point Morrison already fell into the water but you can clearly see her character moving out of the scene in the back and to the left.
http://www.imdb.com/name/nm2604794/
Sadly... I've noticed this haha.
At first I thought it was just her escaping, but then she's underground again, so must def be a goof.
"The battle for the soul is fought in the forum of art."
It's not an error, it was her escaping. If you look, there was nowhere for her to go underground under the dock, there was just land/shore. She got out and ran back to the grate.
When the killer steps on it you clearly see a handle, and when she turns to run into the tunnel, you see stairs that were in front of her, no doubt she used the handle lift open the grate and the stairs to get down there.