Knife in the Water (movie, 1962)

On their way to an afternoon on the lake, husband and wife Andrzej and Krystyna nearly run over a young hitchhiker. Inviting the young man onto the boat with them, Andrzej begins to subtly torment him; the hitchhiker responds by making overtures toward Krystyna. When the hitchhiker is accidentally knocked overboard, the husband's panic results in unexpected consequences. This was the first feature directed by Roman Polanski.