It seemed literal to me. I was left wondering what the point of the movie was.
Except everything else he saw seemed pretty literal too. Like all the furniture suspended in midair because gravity stopped working. That was shot to seem pretty literal. And when his wife saw the storm at the end, she got rained on by the orange stuff he'd been seeing too. So I'm not convinced it was cut and dry that the ending was literal. Actually, as my roommate pointed out, interpreting it metaphorically actually makes the ending kind of a happy ending ( the wife and daughter accept that dad is crazy and begin to deal with that. The wife begins to understand the size of the problem, and even faced with something that catastrophic, her response is "Okay." ). My roommate first took it as literal as well, until I pointed out that there's a strong case to be made that it's not literal. I mean, we're given no cause for the final storm. Particularly the giant tsunami. If it's supposed to be literal, that's the end of the freakin' world right there.