I remember it's an Australian movie with a plot something like this: A guy has to take care of a large, empty house with a swimming pool because the owners take a vacation. The next day there is a girl in the pool and she says that she has the keys and she is some relative. Guy falls in love with her, etc. The plot always involves the guy getting up and unable to determine whether it was a dream or not. In the end he gets locked into the house and his girlfriend is unable to open the doors or windows. At last he wakes up in the hospital, but realizes that he is dead and the evil (ghost girl) is the nurse.
I found it. It's Crush.
An American student studying in Australia mistakenly gets involved with a mysterious and unhinged girl while house-sitting for a rich family on vacation.