Well that's the theory of some people on Reddit.
So, here is what they believe really went down in the movie.
She drowns at the beginning of the film. Danny tried to save her, but she went into a coma.
The entire story takes place inside Sandy's mind as a dream.
Then, as we see Sandy and Danny fly into the sky, in the car, that's Sandy dreaming of her last moments before she dies. Phew!
Sounds likes these theory people have vivid imaginations :)