[bluGill]

In most cases you can drill just about anywhere and find water. So any approach from random to looking at geological features (which you can gather from seismograph data, if needed set off an explosion in controlled locations...) will find water. The latter approach may find a specific place where a less deep well can work, but in most locations there isn't that much a difference (at least not among the reasonable places to drill your well - if you are willing to pay for miles of pipe you can maybe have a useful choice of location).