Having a wonky horizon can make everything in your image look like it's sliding to the right or left of the frame but a quick correction in Photoshop will stop your scene looking like it's about to slip out of frame.
Method One: Rulers and Crop Tool
If you haven't already, make your rulers visible by going to View>Rulers or by pressing Cntrl+R on your keyboard. Then click on the top ruler and drag a guide down to where the horizon level should be. When you let go of your mouse button the line will turn blue.
To correct the horizon go to Image>Image Rotation> Arbitrary and as you've used a guideline, you'll see a figure has already been entered into the Angle box so you don't have to guess how far the image will need tilting to correct the horizon. Hit OK and your image will be cropped so the horizon sits level.
You can ignore the guideline now and remove it by clicking it and dragging it back up to the ruler if you wish. The horizon may be straight but the adjustment has caused white space to appear around the edge of the image so a crop needs to be made to remove the empty space. Select the crop tool and draw out a shape, covering as much of the image as possible without getting any of the empty space in the crop. When you're happy hit enter on your keyboard or right-click and click crop. Your image is now complete.
Method Two: Crop Tool Only
Select the Crop tool and draw out a rectangle so the top or bottom middle anchor point is positioned over where the horizon should be. Don't hit crop, instead move your cursor so it's outside of the box, towards one of the corners so the shapes changes to a curved, double-pointed arrow. This means you can rotate the crop. This next part may sound odd but you need to now rotate the shape you've created until it lines up with your original wonky horizon.
You now need to pull the anchor points on your selection out so the crop covers as much of the image as possible without going over the edges of the image. When you've drawn out your larger box hit enter on your keyboard or right-click and choose crop to apply the changes. Your horizon will now be straight.
For CS5 Users:
If you're using CS5 you can use the Ruler tool to draw out a straight line where the horizon should be and click straighten to fix the image. You'll find this tool by clicking and holding on the Eyedropper which will bring up a small menu.
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