In a nutshell: Swimming pool is a film about an English detective story writer who goes to France in search of inspiration for her new book. As she becomes somewhat comfortable in her new environment, her routine is completely uprooted with the arrival of a mysterious, brazen and sexually assertive young woman.
This intricate film is like an abstract painting with the plot so tentatively outlined it leaves the viewer to fill in the blanks. Swimming pool isn't really about solving a mystery, the director was careful enough to leave many little clues that don't really add up logically. So instead, I believe he was trying to encourage the viewers to focus on the relationship between two women and the evolution of the otherwise emotionally restrained Sarah (the writer) as she observes the young French woman with a mix of envy and contempt.
Definitely not for everyone.