I think discounting underground water reserves is folly. I work in the oil/gas industry testing recovered hydrocarbons (oils, gases, condensates) and various fluids mixtures (natural waters, pumped KCL water, pumped frac oils, pumped acids and other various pumped chemicals for viscosity or inhibition) for content, quality, and velocity, or how much force on surface the reserve has. Usually we deal only with 5,000-10,000 psi wells, but there occasionally are wells with greater pressures on surface, and I've seen some of the bottom-hole pressures at upwards of 70,000 psi on the frac crew equipment) after a gas or oil well has been drilled (However, sometimes they contain just water [called dud wells, as are ones that turn out incredibly lower pressures or content than predicted, and consequently are an oil companies nightmare considering the huge costs to explore, prep, dig, complete, and then maybe abandon the well]). We can only (fairly) accurately see with seismic to about a depth of 7 kilometers. I worked on an exploration well where we went to that depth drilling. It was the only well I've personally worked on that was nearly over 15,000 psi working pressure. A very hairy job, watching 3 inch thick iron pipe [many thousands of pounds of weight] jump around on the ground like it was going to pull itself apart while we flowed it back). I've read of experimental holes dug for scientific purpose up to about twice that depth, so around 14 kilometers.
To put that into perspective:
"Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of Earth."
We can visually "see" what is underground using seismic equipment to about 7km, that gives us a fairly accurate picture for 7k of 2,890km or 0.0024% of the the mantle depth.
Also, we have gone to about 14km of 2,890km, or 0.0048% of the depth.
Seems like hubris, or just plain ignorance, to automatically assume we have a good grasp of just how much water is in there. There may still be plenty of pockets, or aquifers yet undiscovered.
Also keep in mind things react differently, even unexpectedly, under some of the extreme underground pressures. We see this all the time in flowing wells. And water underground begin squashed down by tens of thousands of pounds-per-square-inch of pressure isn't the same as water sitting on the surface at atmospheric pressure, first thing it does when it hits surface if mixed with the right combinations of gas/oil is turn to tons of foam. We have to pump tons of inhibitors and/or diesel (diesel helps cut foam and is easier/cheaper than foam inhibitors) to keep from blowing it out the flare-stacks, down the pipeline, our out the 400bbls. It's a real pain, especially when the frac process pumps CO2 to fracture a well (think shaking a can of soda, heating it to 60 degress celcius [140 degrees for Americans], then opening it fast).
Finally, there are also strange occurrences that just go against the norm when dealing with the depths of the earth. We work through it and get the gas/oil, but even the engineers get stumped by some of the ways the wells behave, some of the odd mixtures of fluids/gasses we recover, or by the science going wrong and giving us dud wells when everything pointed to real 'screamers'.
Anybody putting their faith strictly in science is destined to be let down, that's the only constant scientific fact.
In the end, scientists are just people, and science is just ideas people have based on the current information available, and later on others will change those ideas by new information that comes along, and proves the earlier scientists were either dead wrong, or just didn't have another piece of what will always remain an incomplete and infinite puzzle.