Sleep is Lovely(1968)

Peter can't get over his ex-girlfriend Elsa even though they broke up over a year ago. He spends all his time on a barge owned by his friend John and John's younger brother Colin. One morning Peter, John and Colin see a middle aged man, Clive, fall out of a motor cruiser into the water. They rescue him and decide to ransom him for £1,000. Peter and Elsa are-reunited but Elsa then commences an affair with Colin. Clive turns out to be Elsa's father.