For what it worth, perhaps SOME dowsers are akin to so-called "mentalists". Once thought to possess supernatural skills such as mind reading etc.. most folks these days realize that "mentalists" have highly developed observational skills. In popular literature, the Sherlock Holmes character is an example. On contemporary television the drama called the Mentalist is another. In the Mentalist, the main character is a skilled performer who had a popular Vegas act as a telepathic type. After his family is murdered he realizes his life is a fraud and becomes a police consultant. He makes it clear to the police that his "gift" is simply all about putting what he sees into context, picking up on physical reactions of people, being hyper observant and using logic. Granted,he has more highly developed skills but so do card counters in Blackjack. A summer film on a similar theme called "Now you See Me" is due out in June.
So, it is logical to argue that some successful water seekers simply see things you and i miss...subtle changes in elevation, where certain mosses grow, slight evidence of natural salt licks and slight animal tracks. Nothing supernatural there, just things modern man has forgotten.