She didn't die!
I came here to read what people thought of that beautiful ending and I was quite surprised to learn that everyone seem to be convinced that Susan dies in the end.
if you watch the scene again you can easily see that right before she loses herself in the water, she looks back and playfully smiles. she does that twice in fact. I'm pretty sure the director wanted to make sure it's not missed as the wrong interpretation would turn this scene into a very morbid ending as it has been for many apparently.
yes she is injured but that lamp didn't even hit her head hard enough. for a death causing injury it would have been very ridiculous scene. Susan fakes her death playfully to tease mike. she enjoys the innocent attention/ obsession she gets from him.
I thought it was one of the most beautiful endings I had seen.