I don't see a whole lot of wiggle room on the "protagonist is dead" angle, honestly. I mean, yes, you can contort things a bit, close one eye and squint and say maybe he's time traveling or something, but it's pretty clear what implication you're supposed to draw, particularly with the repeat emphasis on the line at the end of the story. (And also the water, as passing over such a barrier is a feature of many afterlives, and the stillness, and the gradual fading of the clinging vestiges of warmth and light. Plus, in comparison with "What Happens When You Wake Up in the Night," there is a clear and plausible mechanism by which the protagonist could have died, as well as external confirmation that what he perceives is not what everyone else is perceiving.)
However, this is a very well-done version of Dead All Along, one that keeps its cards close to its vest until the reveal, at which point it shows just enough for you to get the *click* and then ends before anyone can go, "Aw, man, that's it?" It doesn't even reveal the twist; it reveals up to the point just before the twist and lets you take the last step, which is why it's such a powerful sensation for many of us. (Which is another reason I don't see him not being dead, by the way; if the story is meant to be unexplained, like "What Happens..." then it should belabor the point a bit more and provide more oddities. The only reason to end just as the main character realizes what's happening is if you want your audience to draw the obvious conclusion themselves.)
I dislike Dead All Along, but this is a good story and a fine exemplar of the genre. I enjoyed it and appreciated the skill with which it was told, both in the sense of the words on the page and the nuance of tone in the reading.