The basic rule of thumb when processing images is, “Correct major errors first and minor errors later”. One of the first major corrections we perform is aligning the dominant vertical or horizontal lines in an image.
You should, of course, take care to photograph the horizon so that it appears horizontal, but you will nevertheless need to make corrections every now and again. Make this your first step, as it nearly always involves a subsequent crop.
You can rotate your image in one of three ways:
Use Image ▸ Image Rotation ▸ Arbitrary. Judging the correct degree of rotation can be tricky, but you can first drag the Ruler tool () along ...