There were definitely some very frightening scenes in that movie. I found the first scene with the woman swimming at night really horrifying when I finally did see it, many years after the movie came out.

There was a much less known movie about 2 scuba divers, a married couple, who get lost at sea off Australia when they are inadvertently left behind while diving by their tour boat. I think it was called "Open Water", and it was really harrowing. One did not walk out of the theatre after that one feeling too good at all.