I didn’t watch it, I haven’t even heard about it, but I looked it up on Wikipedia and I guess the end is supposed to be ambiguous and there’s two different interpretations. One being Sarah was alone at the villa the whole time and Julie was a fictional person she created in means of her book. The other being that Julie was a younger interpretation of Sarah. It’s described as an “French-English psychological thriller and mystery film ” and seems to me as if the character Sarah was having problems distinguishing fantasy, as related to her book, and reality. Hope that helps, if you want to check out the article for yourself here’s the link: http://en.wikipedia.org/wiki/Swimming_Pool_(film)