I Know What You Did Last Summer

In 1996, Four high-school seniors - Julie, boyfriend Ray, best friend Helen and Helen's boyfriend Barry - drive home from a party. While driving, Ray becomes distracted, hitting a pedestrian. Another teenager, Max, stops nearby. Julie convinces him everything's okay; he drives off. The group decides to dispose of the body. At the docks the man revives, attacking them before falling into the water.
A year later in 1997, Julie's home from college for the summer. She receives a letter stating, "I know what you did last summer." She tells Barry and Helen about the note. Barry suspects Max. The trio go to the docks where Max works as a fisherman. Barry threatens Max with a hook. Julie discovers that Ray works there; he tries reconciling with Julie. Max is killed by a figure in a rain slicker wielding a hook. Barry discovers a note in his gym locker containing a picture of his car and the message, "I know." Going outside, Barry is almost run over by the slicker-wearing figure, driving Barry's car.
Julie reveals that the person they hit was David Egan. Ray still suspects Max. Julie and Helen think the killer may be a relative. They head out to the Egan's home. They find David's sister Missy, who explains that David's death devastated their family. Missy tells them that "Billy Blue" paid his respects after David died, claiming to be his friend.
That night Helen prepares for the Fourth of July parade. Max's killer sneaks into her house, cuts off her hair and writes, "Soon," in lipstick on her mirror. As Julie goes to Helen's house, she finds Max's corpse in her trunk. When she goes to show the others, the body's missing, causing Barry to think she hallucinated. Julie, Helen, and Barry confront Ray about recent events. Ray claims that he received a similar letter.
As Helen and Barry participate in the Fourth of July parade, Barry notices people wearing the same kind of slicker. Chasing one, Barry leaves Helen on one of the parade floats. As it passes by a building, she notices a shadowy figure in a slicker wielding a hook threateningly.
Julie revisits Missy. Missy tells Julie that David left a suicide note. As the writing matches that of the note she received, Julie tries convincing Missy that it's not a suicide note but a threat. Missy orders her to leave. At the annual Croaker Pageant, Barry is killed while watching from a balcony. Helen rushes to the balcony with a police officer and finds no sign of the killer or Barry. The officer offers to drive Helen home.
Julie researches David Egan's death. A year before the accident, he and his girlfriend Susie were in a car crash near the scene of the foursome's accident. David survived, but Susie died. The research now mentions Susie's father, Ben Willis. Julie believes they ran over Ben, who killed David. While driving home, Helen and the officer are stopped by a stalled truck. The officer is killed by a dark figure with a hook. Helen rushes to her family's store, where her sister Elsa lets her in. The killer enters through a side door and kills Elsa. Helen manages to evade the killer by making it to the second floor and jumping out the window, but she is eventually killed, too.
Julie goes to the docks to tell Ray her theory but is disbelieved. She notices the name of the boat he's working on: Billy Blue. Julie runs off, mistakenly believing Ray is the murderer. Ray pursues, but a fisherman stops him and puts Julie on his boat. Looking around, she finds a room containing photos and articles about her. Julie realizes she's on Ben Willis's boat. Ben sets the boat adrift. Ray catches up to Ben's boat with a motorboat. He uses the rigging to cause Ben to lose his hook-carrying hand as he plummets over the side. All that's found of Ben is the severed hand with the hook. When the police question Julie and Ray, they deny knowing why Ben attempted to kill them.
A year later in 1998, Julie is two weeks away from returning home to see Ray. As she enters the showers at school, she notices one of the steamy glass doors has the sentence "I still know" written on it. A dark figure crashes through the door.

Question: who realizes she's on Ben Willis's boat?
Answer: Julie