Definition: A pond, lake, river, stream, sea, bay, ocean or any other open water on the course, including ditches and drainage ditches.
Did you know that there doesn't have to be water in a water hazard? If a seasonal creek, for example, is defined as a water hazard by the committee, but your ball finds it when it's dry, it's played under all the rules for water hazards.
Also, water hazards extend vertically and their boundaries should be defined by yellow stakes or lines. Those boundaries often extend out a few feet from the surface of the water itself. If your ball crosses the marked boundary but sits on dry land, it's still considered in the water hazard.
The stakes or lines defining the hazard are considered to be in the hazard.
See Rule 26.
