It's likely you're thinking of "La Piscine" (1969), also known as "The Swimming Pool" in English. This film stars:
* Alain Delon as Jean-Paul
* Rommy Schneider as Marianne
* Maurice Ronet as Harry
* Jane Birkin as Penelope
There was also a remake in 2003 called "Swimming Pool", starring:
* Charlotte Rampling as Sarah Morton
* Ludivine Sagnier as Julie
* Charles Dance as John
* Peter Coyote as James
Could you clarify what you're looking for?