Sometimes, for example when you scan a photograph taken by a drunken photographer (watch out, Erik!), the horizon isn't really horizontal.
When you take the Measure Tool and draw a line that follows your horizon and then choose Image>Rotate canvas>Arbitrary, and then click ok, your horizon will be made horizontal.
When you take the Measure Tool and draw a line that follows your horizon and then choose Image>Rotate canvas>Arbitrary, and then click ok, your horizon will be made horizontal.