I lesvia (1975)

A Young evil woman and her older rich husband take a trip to London, so she can have a good alibi while her plan takes place. She sends her young lover Alexis to seduce her husband's daughter and kill her, so when they would return from London and her husband would find out, he wouldn't handle the shock and die from a heart attack, letting her alone inherit everything. Christina the young heiress stays with their maid Mary, at their country house having vacation. Mary is a lesbian and tries repeatedly to allure Christina into her Sapphic games but she won't indulge, cause she prefers Alexis. Alexis starts to fall in love with Christina and has second thoughts but he then gets a call from his girlfriend, and he decides to execute the plan immediately so he takes Christina on a boat for a night fishing trip experience but things don't go exactly as planned.