For the most part, though there are no guarantees at what depth you will hit water, the rate of flow, or the quality of the water when you reach it - the level at which the local aquifer exists varies wildly. Wells can go upwards of over a thousand feet down without finding water. In some places, because the drilling can actually clog aquifers, they'll try hydrofracking to increase rate of flow. And sometimes, people are lucky - they have artesian wells that provide an abundance of water that naturally flows to the surface thanks to the rock formations in that area.

Before a driller gets started on a well, they'll check local water tables, the geology of the area, examine previous wells drilled in that neighborhood to determine at what depths water was discovered, what the average well depth is, and ensure that Call Before You Dig has been out to mark the area with possible underground hazards (cable, power, gas, fiber, sewer, etc). On occasion, they'll blast if drilling where the bedrock is particularly thick (such as granite).

My father and grandfather worked in the water well industry for over 30 years and both have agreed that while dowsing is by no means a perfect process, it can work in the correct hands. A lot of times, my father was able to look at an address and determine the approximate depth the well would need to be, take an estimated guess at the quality so that he could then design a pump and storage system to maximize the well's production.