The exposure is excellent!

I do not think the top of the falls is too centered. There is a natural draw for the eyes from top left to lower right or vice-versa and that line leads to the falls in either way. Centered subjects aren't always against "The Rules"
I will say that I wish there was more vertically, especially at the bottom but I realize that whatever is there might not be as attractive or possible to shoot. It appears you did shoot vertically and cropped, which makes me wonder what's there.

The second also has beautiful exposure and is a lovely shot. It does seem rather cramped, though, at both top and bottom. What lens? Perhaps you couldn't get any wider here.