Slack Bay (2016) on Netflix

Plot summary

Summer 1910 on the coast of Côte d'Opale. The Van Peteghems, wealthy industrialists, spend the summer in their mansion above the bay, where the Slack river joins the sea only at high tide. Across the creek live the Bruforts, fishermen and oyster farmers who ferry people across the water, led by the father nicknamed The Eternal. André Van Peteghem, his wife Isabelle, his sister Aude and Isabelle's brother Christian are among the family at the villa. Visitors to the bay keep disappearing, and inspectors Machin and Malfoy trace the cases to this stretch of coast. Ma Loute, the eldest Brufort son, aged eighteen, and Billie, the eldest Van Peteghem daughter, meet, and over five days the two families cross paths while walking, sailing and bathing.