Can you draw a straight line along the horizon with the ruler tool below the crop tool?
Four ways- (In the CROP tool)
1. Click on the "Level" tool and use mouse to draw a line along the existing un-level horizon.
2. Click on "Auto" (in LRv6) Lightroom finds horizontal ( or verticals ) to auto rotate.
3. Move the angle slider.
4. Type in a value for Degrees rotation (- minus for anti-clockwise)