This reddit comment goes into detail on what the photographer did:
Here's a summary of the settings, technique and post processing used for the photo, taken from a comment on my wallpaper post a month ago:
Canon 5D mkII DSLR + 28mm f/1.8 USM lens. Filters are extremely important for this shot. We shot pre-sunrise at a muddy/sandy area on the coast about half an hour from where I live. Plenty of mangroves and a few trees out in the open like this one that are mostly above ground but start to flood at high tide. I wanted to smooth over the water as much as possible which requires an ultra long exposure, but shooting straight into a bright sun means I needed to use a huge amount of darkening with my filters. After composing the shot without the filters (too dark to see through the viewfinder once they're on), I stacked an ND400, an ND8 and two ND4 grad filters on the front of the lens. The NDs helped slow the shutter speed and the grads helped darken the sky for a more pleasing effect. It took a few attempts at getting my exposure right, but the final result was shot at ISO50 (keeping sensitivity down), f/8 (good sharpness on this lens) and 70 seconds. To get any shutter speed past 30 seconds on a DSLR you need a remote switch, I picked one up off ebay for 4 bucks.
Post processing: This is the one photo that everyone asks or assumed is heavily processed, but it's had hardly anything done to it. I corrected for the red tinge that comes from the cheap filters and added some contrast and blacks for the tree silhouette, and that was it. It's great to see that in the end, nothing beats a photo that was constructed well in-camera. Here's a comparison of original RAW file to final copy.