The rule here is to keep this bad boy out of the center of the frame unless you handle it skillfully. One of the reasons that we try to avoid centering the horizon is that is splits the frame in to two equal parts that can compete for the attention of your eye. By placing the horizon in the upper or lower third of the frame, it allows the eye to travel around the image. You can also go to the extreme and place the horizon near the top or bottom of the frame.
When can you break this rule? With reflections, like in a lake, pool or other bodies of water. See link below: