I'll bet that Channing Pollock would give you a very different testimony about this sort of skill... it's the source of one side of his wealth.
The so-called "Tests" by the Scientists always involve sitting water vs. naturally flowing springs which is one of the biggest reasons for the failure rate in the lab setting. There have been several explanations for this, even within Dowser's groups that refer to one's intuitive understanding of how the lay of the land, vegetation, and other such subtle indicators kind of direct the operator to a happy find. Mind you, this is just one theory behind the fact. The problem the scoffers have however, is explaining the unusual and "obvious" success rates throughout history, in that dowsing has been used for thousands of years as a means for humankind to not just locate water & minerals but even potentially bountiful grazing and farm lands, etc.
Well into the 19th century Dowsing was very much a key part to Survey Work with aspects of it tied to other legitimate fields.
Yes, I personally believe that magnetic fields have a lot to do with the success some have over others; some are more sensitive to such influences for whatever reason. But my support of this particular theory stems from the lore tied to LeyLines, which are oft time described as a subtle electro-magnetic energy (a web) that crisscrosses the globe. Even in such occult lore the idea that smaller and smaller leyline fields exist within the boundaries of the larger help sustain some of the contemporary findings tied to this course of practice. I'm not saying that "this is it" only that it is one of the proposed theories as to how dowsing may work and why; one that I willingly embrace in that all folklore and myth has a factual foundation somewhere along the line that gave impetus to the more dramatic (exoteric) perspectives.
It is impossible for those that have already made up their minds as to what it what, to give a skill of this kind an honest evaluation; the believer will only see what supports that faith while the cynical does the same, forever manipulating and devaluing results so as to prove himself "right". The truth being that neither finding is even remotely accurate by way of assessment and yet, neither is entirely wrong. Finding the points of agreement however, that's the rub and that is where 99% of human being fail, regardless the issue.