Water witching, also known as dowsing, is a type of divination employed in attempts to locate water underground.
Dowsing is also known as divining, and has its origins in Renaissance magic in Germany, and continues to be popular among believers in Forteana.
First, a couple of observations.
You will always strike water if you go deep enough.
There are natural land features that are likely to indicate water at a reasonable depth.
Given those two facts, there’s no evidence that dousing works any better than guesswork by a knowledgeable hydrological amateur.