Why is that? “Water masking” or “masking” in general refers to blocking out information that is present in an image but that is not shown. It’s called that way in Photoshop when you mask a layer, it called like that in movies when the top and bottom parts of a Super 35 frame are masked to hide mic booms and such, and it still applies here because the texture information of shallow water is available but masked out/invisible in certain areas.