Horizons usually look better in photos if they are straight. And I mean dead straight. There is nothing worse than a slightly crooked horizon - it just looks like a mistake.
Now if I said that I always shoot my horizons straight I would be telling you a giant porker - the truth is, they are rarely completely straight. Luckily, horizons are easily fixed in Photoshop.
Here is an image with the horizon slightly higher on the right-hand side.
First, find the ruler tool (it hides under the eyedropper tool).
With the ruler tool draw a line along the horizon.
Then go to Image > Rotate Canvas > Arbitrary. Press OK.
Your image will tilt. Now crop off the white bits and voila, you have a straight horizon.
Of course - rules are made to be broken. Tilted horizons can be fun but the mood of the image needs to suit - they work best with images with high energy, movement and excitement.
And remember, if you are going for a tilted horizon effect, make sure it's really tilted.