It seems like specifying a contract where you're going to pay for the well digging and he gets as many tries as he wants to select well sites isn't likely to lead to a good outcome whether he's a dowser or a geologist. Pay for performance seems like a lot better model than pay for consultation in this instance. Of course, I dare you to find a dowser who would actually agree to that kind of contract, heh.
Better still is payment based on past performance. Whether he's a dowser or a geologist, how many times in the past has he succeeded as a fraction of his attempts? If dowsing is a crock (and I think it is) and study of geology actually improves the probability of finding water, then the geologist should win over time. Unless, of course, the dowser has actually acquired an intuitive sense of geology, and the dowsing rod is just a prop.
Of course, I doubt you will find a dowser who is willing to compare his success rate to a geologist.