These people do work in the desert. I have a buddy that drills wells here in Arizona and he WILL NOT put a drill into the ground without a Witcher. I have a really hard time with "witching" from a logical POV, but my buddy and his $130k truck/equipment swear by it. I call that putting your money where your mouth is.
The test would be finding a spot where a witcher says there isn't water, and then drilling there.
There's water almost everywhere in the ground -- the questions are is there enough of it to meet the need? is it usable (ie: not contaminated with salt or toxic chemicals)?