If you haven’t watched a Knife in the Water by Roman Polanski, I recommend you do so. Here is a link to the film (on a shady Russian site).
A man and his wife are travelling to the coast of a lake to sail, they almost run into a young male hitchhiker, and after some bickering and driving, they all decide to sail together.
The relationship between the younger and older man becomes rocky, and they try to fight for dominance in various indirect ways. Eventually, they get to a tense point where the older man foolishly drops the younger man’s knife into the lake and angers him, prompting a fight. The young man falls into the lake - though they are not sure if he can swim. He doesn’t resurface, and they search for him, but fail. The wife then gets mad at him, and he leaves her be.
It turns out the boy did know how to swim, and he swam back to the boat and met up with the wife. They had a small fight, made up, and then proceeded to cheat on the husband together. Afterwards, the boy left the boat at took off on his own, and then the wife met up with the husband. After meeting up with him, the husband had thought about the incident and planned to drive to the police to report the death.
When they drove to the police station, the wife asked him if he was nervous; after a conversation he admits he is, and she proposes not to go. He refuses, and is adamant about reporting the drowning. The wife then tells the truth: that he resurfaced, they kissed, and left before she met him. He brushes it off as a lie, but then he stops the car and starts to think for a long time.
A great ending. Basically, the husband is stuck, and cannot choose which lesser evil to take: drive home and never know if the wife cheated, and potentially risk her independently reporting the incident to the police (if the boy died, that is) or to go to the police and find closure with regard to the cheating, but risk going to jail.