In an analogous way to peak oil, peak water refers to a peak in the production of usable water. There are differences, sure: water is mostly renewable. But where water is essentially non-renewable, such as with fossil groundwater, the analogy is spot on. Where water is renewable, the analogy is still very useful. The peak denotes the point where the production of water outpaces the natural supply; if this practice is sustained, the water stored will not be.
There’s another concept out there, too: peak ecological water. This refers to a the turning point in water production when any further water production starts to cause more harm than good. This is much harder to quantify than peak water, but it still apparent.
Here Peter Gleick, president and co-founder of the Pacific Institute, provides a more thorough explanation.