Knife in the Water (1963)

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.