Why it masks so well
Falling water produces energy across the whole audible range at once, like pink noise with the high end filled in. That density is what covers speech, keyboards, and a television at the same time. It is the sound to reach for in a loud flat or a shared office when brown noise is not enough.
The cost
Density is fatiguing. Waterfall at the volume needed to cover a loud room is a sound you will want to turn down after an hour. Use it for the block that needs it, then step down to rain or brown noise.
Volume
Start low and raise it only until the intrusion disappears. Waterfall is the sound most often set too loud.