Okay, I'm a geologist. It happens that I live in an intermontane basin filled with alluvium, and I know the water table is about 30 feet down at my location. If I were so inclined, I could take a couple of wires or a willow stick, walk around a bit for show, then "find" a place. I'd tell you to dig 30 feet and you'll find water - and I'd be right. The knowledge this takes is not that hard to acquire, especially if you want to work in a specific region. I suspect many of the "professional" water dowsers are simply doing that and making a buck from credulous buyers.
That said, I have seen people do some freaky things with dowsing rods. As a scientist I have to doubt any mystical source, but I admit having had a few WTF moments courtesy of one old fellow I used to know. He would find ore veins - where I knew they actually were, and he couldn't have because I hadn't shared my survey findings. But guess what? Ore veins do affect both the magnetic and gravitational fields. I don't completely discount an ability by some people to detect that - after all, some birds apparently do.