It all depends on.. the amount of water you need, the quality of water, contamination by man or natural. If you need clean drinking water it's a good idea to go through a couple aquifers, to find the cleanest possible water. You could driĺ and hit water pit also a methan pocket at the same time, don't want to drink that. Over time local drillers have accumulated knowledge on there local geology and have a very good understanding where water is.