Water is contained in just about every distilled spirit. Water plays an important role in the final flavor of spirits due to the trace amounts of minerals contained in the water. Distilleries are often at the same location for hundreds of years because of the desire to be near a quality water source.
Typically, a high calcium content in water is considered to be desirable in water, while a low iron content is also preferred.
Water comes generally in five sorts: tap, "spring", distilled, de-ionized, and reverse osmosis (RO).
- Tap water, of course, is the stuff that comes out of your tap. It will vary depending on your location, the time of the year, the recent weather, and any problems your local municipality may be having. It will contain various dissolved (and added) substances that may, or may not, react with your wash/mash/etc. You can get an analysis of your tap water from your local utility.
- Spring water comes out of the ground. Therefore, you get whatever comes out. While usually purer than tap water, still a grab-bag. Usually tastes pretty good, though. Some swear by it, and now, at least in the US, many groceries sell it by the gallon.
- Distilling water removes all the solids (salts, tannins etc.) but may still contain things like sulfur compounds that are not very nice.
- De-ionized water is anything but. It is made by running the tap water through ion-exchange resins. Yes, that was "ion-exchange" not "ion-removal". "De-ionized" water contains just as many ions as it started with--just different ones! It is common, in producing de-ionized water, to use a carbon filtering system, also. Carbon will remove a multitude of sins, but may also add some of those nasty sulfur compound, unless it is a very good grade.
- Reverse osmosis water is probably the "cleanest" you're going to find. It is filtered under pressure through a very selective membrane that excludes even such things as chlorine ions. As you can imagine, it's pretty expensive, as water goes. Best place to find it is at aquarium stores that cater to salt-water enthusiasts. Their expensive fishes need a carefully balanced environment, and RO water is the best place for them to start.