By vertical, do you mean the trees? One of them still seems a little tilted. Normally, water is always approximately 'tangent' to the Earth, so it's best to keep it straight horizontal no matter what the other stuff looks like -- if the other stuff is slanted, you can try and balance it by having slanted stuff in the opposite direction in your image as well.
But maybe you're talking about the curvature of the lake's surface? If so, try "Lens Correction" to adjust the distortion caused by the lens.