Use the Distort>Lens correction filter.
I temporarily added canvas area to the bottom of the picture so that correction would affect the horizon line. (There was very little change when leaving the horizon in the center.)
Move a grid line along the horizon for reference.
Level the horizon, using the angle setting, then
Flatten the horizon by adjusting the Remove Distortion slider.

Recropped image after correction.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=27970391
original image by PhotonFiend: