Eaux profondes aka Deep Water (1981) - Michel Deville

On the island of Jersey live Vic and Melanie, a couple who appear well integrated and happy. Melanie enjoys openly flirting with other men under the calm gaze of her husband, who sometimes claims to be responsible for a man's disappearance in order to scare off his rivals. But this tolerance is only a facade. When Melanie brings home a young man named Carlo, Vic ends up drowning him in the swimming pool during a party. The investigation rules the death an accident, despite Melanie's accusations. Later, when another potential lover, Cameron, catches Melanie's attention, Vic lures him to a secluded place, kills him, and throws his body into the sea. The case is closed for lack of evidence, and Melanie, realizing that Vic killed these men out of love-or to win back her affection-reconciles with him.