The Horizon line tool allows you to quickly separate Sky from Water and Sky Reflections. Once the horizon line is set, no pixel below it will be labeled Sky and no pixels above it will be labeled Water or Sky Reflection.
The horizon line is also used in the lighting calculations, in computing sky reflections, and in the Straightening tool in the Fixes section.
Click and move the mouse up and down to match the Horizon line to the horizon in your image or where the horizon would be if it is hidden.