On the corrected image the horizon seems nearly horizontal, however the dock still isn’t. This aspect strikes me most from this very nice image.
The only valid horizon we can find in a landscape is the sea (or a sufficiently large lake) horizon, and still is a straight approximation of what actually is a curve; unless you are a terraplanist, then it is exact.
Regarding the dock is like Slobodan explained. You are assuming the camera was oriented orthogonally to the dock's end, and in that case the dock's end would be parallel to the horizon (which we don't have BTW). In case the camera was oriented in any other direction, the dock's end would never be parallel to the (non-existent) horizon.
However, I think the original question is a bit silly. Here is my visual argument:
I assume you are just making fun Eric. In case you don't, the original image has a good bunch of trees which in average tend to grow straight. Statistically it's highly unlikely that all those trees made some arrangement to grow for years inclined towards a specific direction in order to fool us today. So it's quite safe to conclude that the distortion was caused by camera tilting and can easily be reverted.
Regards