Open water is relative. Generally it represents water with no clear cover above the water but it also relates to the distance between the surface and the element you are fishing (weeds, humps, etc.).
Example. Fishing submerged grass which is just a few feet from the surface not open water to me. The predominant element is the weeds and not the water. In other words the gap between the surface and the weeds is marginal.
A submerged hump in 33 feet of water that only goes up 5 feet gives me 28 feet of open water. Fishing a spook over this hump would be considered open water fishing.