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Exactly what the title says. I don't know to exact pH level of the water but it's at a level that it would kill a human should they drink it.
What/how would a creature evolve to deal with drinking water with a high acidic content? (Or how would their body be able to neutralise it?)
The stomach of actual species is already tolerant to low pH, as it has to handle gastric liquids. The intestine is not resistant to acids, and the content of the stomach is neutralized by basic substances which balance the pH.
A similar approach can be applied to your fictional species: basic solutions can be used to neutralize the pH of the ingested water, and the section from the mouth to the stomach can be covered with the same acid resistant mucus which covers the stomach. The resulting salts would have to be expelled by the body through urines or feces.
Another option would be for the species to swallow small limestone stones/cobbles together with the water. These (or similarly reactive elements) can also neutralize the pH. The produced gas would force your animal to burp some more. This can be a useful element for your plot (Tirannosaurus burpus...).
Limestone is mostly made up of the mineral calcium carbonate (CaCO3). This is not very soluble, so rocks don't dissolve very quickly. But if you add an acid, you add hydrogen ions (H+), which will react with the carbonate to form hydrogen carbonate HCO3- ions, which are very soluble in water, and the limestone will dissolve. Or, if there is more acid, two hydrogen ions will react with a carbonate to form carbonic acid - H2CO3 - which will decompose to form carbon dioxide - CO2 - which eventually bubbles off into the atmosphere, and water H2O.
Look, evaporation and rainfall makes an extreme pH scenario unlikely. But if you are set on this, it is down to proton pumps and other chemistry. Some chemical reactions (the sulfur cycle, the nitrogen and carbon cycles) become linked to allow metabolism in an acidic environment.
Some parts of earth are highly acidic. Others highly basic. Life does fine there. It evolves there. You wouldn't do fine with it, because you evolved at pH ~7.
One could ask "how would we evolve to drink neutral pH water." The reason you don't ask that, is because it is what happened. But it happened because of evolution. The other pHs are no better or worse.
Here's a link to the ecology in acid mine drainage, which can reach pH 1.
L.Dutch has a very nice answer about the direct pH issues but do also be aware that water with a low pH mobilises (read dissolves) certain elements that might otherwise remain solid either in or on the surface of soil particles. Iron, Aluminium and Molybdenum can reach toxic levels in groundwater with a pH below 4 (clover can apparently lower local soil pH to 0.8 or less in soils with very low nitrate). While drinking pH 4 water won't necessarily do you any direct damage the dissolved metals in the water will do serious harm just the same.
