Knife in the Water - Roman Polanski

An attractive young woman, Christine, takes the wheel of a car belonging to her partner Andrzej, a sports columnist. While he criticizes her driving skills, a reckless student leaps out in front of the car. Andrzej gives the student a ride and invites him to accompany them on their yacht trip. Attracted to Christine, the student becomes increasingly clumsy. He spends his time jabbing the point of a pocket knife rapidly between the fingers of his other hand. Andrzek argues with the student, who topples overboard, vanishing into the lake. Andrzej dives in to save him, though he fails to find the student and swims ashore. Despite an earlier claim that he couldn't swim, the student manages to hide behind the nearest buoy.