how to correct curved horizon with PSE 9

I am unable to correct curved ocean horizon generated after stitching for panorama. I have tried using filter-distort-liquify after selecting the image. The preview which now works but the change has no visible difference in before and after view. Also I am feeling that liquify tool is not suitable in generating a straight line for the horizon.

Any suggestion or alternative approach option is appreciated. This condition is easily noticed for stitched frames.

Thanks,

-Joydeep