The Snorkel.

On the Italian coast, writer Paul Decker has grown unhappy in his marriage and executes what appears to be a perfect murder of his wife. While Paul is believed to be writing a book in France, his stepdaughter, Candy, suspects him of murdering her mother, as well as her father years before. With the police unwilling to investigate any further, Candy sets out to confirm her suspicions and take Paul down herself.