Leon has it right. The thing that you can't really tell from the original image is that the sky and the sea overlap very slightly. I was kind of troubled the way the sky maintains a consisted pattern all the way to the horizon (something I, for one, do not see very often), but when I looked more closely there was a definite overlap. Which led me to conclude there had been some manipulation at work.
I've attached a full res detail; you can't see it brilliantly well, but you might be able to make out some clouds in the sea.
Leon, good point re it not necessarily having to be digital manipulation.
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