How to straighten the horizon in an image using the Lens Correction filter in Photoshop
So the horizon is not quite as straight as you may had hoped it to be… well, go to Filter>Distort>Lens Correction.
Use the Straighten Tool (A) down the left side of the filter to draw a line across what should be level.
This tells Photoshop how much it needs to adjust the horizon back to its normal self. The image should rotate automatically.
After levelling the horizon, use the Scale option at the bottom to enlarge and remove the canvas around the edges.