This movie was such a breath of fresh air. I spent the whole thing guessing what the twist was going to be. You know what's more fun than doing that? When the film still surprises you. Really...bravo.
For those who watched this and didn't quite understand it, here's what I got out of it: You know how the movie references the Greek myth of Sisyphus? He "made a promise to Death that he didn't keep," and was then forced to roll a stone up a mountain only to have it roll to the bottom once he reached the top. This was his punishment for all eternity. She's Sisyphus!
At the end, when she's standing outside of the wreck (you'll notice there was only one of her bodies at the wreck scene), the cab driver offers to give her a ride. Well, that's Death, yeah? At the harbor, he asks if she's coming back. She replies, "I promise." Only, she doesn't come back, does she? She decides it's better to go try again. To save her son. Only, when she wakes up on the ship, she can't remember what actually happened-only that something is terribly wrong. Hence: she's about to start pushing the rock up the mountain again.
I thought this move was pure genius. It makes a hell of a lot more sense than her just being insane.