I have a couple of vacation photos where I'm not sure whether the horizon should be straightened or not. There are really nice clouds in the shot, and they form a horizontal line across the image that isn't parallel to the line of the land. If I straighten to suit the clouds, the land is not straight and vice versa. It's no big deal but I was curious to see what others do with such a shot. I'd welcome any comments...
21,024 posts I'm late, but my opinion: Yes, definitely, it's the right move to level the horizon.
Thinking a bit outside the (crop) box, you COULD warp the sky so that the cloud tops look more level...
This is probably more extreme than necessary, but I wanted to make it obvious.
-Noel
Well, the surface of water is undeniable level, sure. 
The problem is when—as in this particular example—you can't discern where the level surface of the water is.
Assuming we're looking at a lake in this image, if the far edge of the lake is not a straight line that is exactly perpendicular to the lens, then that line should not be straight.
Imagine an extreme situation where the visible far edge of the lake on the left is closer to the observer and the lens while the lake extends farther away on the right-side of the image so that said right point is located much farther away, then the shoreline would appear to rise from left to right.
A similar effect affects the overall line of the clouds, as those on the right are closer to the lens than the ones on the right.
In this particular case, with no clearly vertical reference, and without the benefit of having observed the scene on the spot, I would tend to trust the atmosphere more as far as providing a natural horizontal line across a short segment at the bottom of the clouds smack in the middle of the image.
The sloping middle clouds just look unnatural to me.
I would say that in this case, it boils down to an artistic decision on the part of the photographer.
That is my personal judgment.
station_two wrote:
Well, the surface of water is undeniable level, sure.
The problem is when—as in this particular example—you can't discern where the level surface of the water is.
...
The sloping middle clouds just look unnatural to me.
I would say that in this case, it boils down to an artistic decision on the part of the photographer.
That is my personal judgment.
I agree on all counts! Thank you.
Bill,
Your interlocutor misses the difference between horizontal and vertical references. My post specifically addressed the lack of any horizontal reference as far as the water in that image is concerned. Your comment in your reply was also very specific about horizontal lines in water in general.
Of course that image is straight! I straightened it myself! 
Yes, I was referencing the OP image as not having any clear horizontal reference. The trees are also not a guide in the vertical direction, but that wasn't mentioned at all in my post.
A Cumulonimbus cloud does tend to have a fairly flat bottom dictated by the layers of pressure in the air of the atmosphere on which it rides and can be used as an indication of horizontality:
Other types of clouds are not useful in that regard.
