Straighten up while cropping
If you’re cropping an image you can straighten up the horizon at the same time using the Crop tool. Hit C or pick it from the toolbox, and then drag it out over the image, deciding on the parts to keep and those to lose (greyed out, below). Next, position the cursor outside the bounding box, and click and drag to rotate. When you do so, you’ll see a grid overlay and all you need to do is line up the horizon with a line, then click the tick.
Cropping Mark’s scene to a squarer format improves the composition and leads the eye through the picture.