Water is both added to food as an ingredient and used to transfer heat for cooking.
For both such purposes, the water should be generally drinkable. Water is a common source of disease organisms. Boiling water for about 5 minutes should take care of this problem. Baked goods containing water also need to be kept at a boiling temperature to prevent the spread of disease.
Water-based cooking methods include:
Water for fermenting 
When adding water to a fermented food, it is generally best to use water with little or no chlorine. Chlorine is added to water to kill harmful microbes such as bacteria, and as such, can change how well a fermentation progresses, by slowing it down or even by ruining it.
Commercial yeast ferments like beer are not as sensitive to chlorine as yeast / bacteria ferments like sourdough, but often still benefit from chlorine-free water as it also affects the taste of the final product.
Chlorine can be removed from tap water by boiling for a few minutes and leaving to cool. Also, leaving uncovered overnight (e.g. in the refrigerator) will reduce chlorine. However, some water suppliers are now using the more stable chloramine instead, which cannot be boiled out and will remain stable in water for longer periods. Many water filters will remove both chlorine and chloramine.
Potable means "drinking" quality. Making water potable involves several steps.
- Remember, some contaminants cannot be removed except with a reverse osmosis filter, for example, heavy metals, or poisons that, in trace amounts, are still toxic, like dioxin.
- Gross physical contaminants should be removed, by pulling them out, coarse filtration with screens or sand, or settling in a quiet pond.
- Fine physical contaminants (haziness or cloudiness) may be removed by fine filtration, filtering through activated charcoal, or by settling with agents, such as diatomaceous earth.
- Biological purification should be performed on any natural water source on the surface. This may be chemical, with chlorine bleach or iodine (will stain clothing), or boiling for 5 minutes at an actively rolling boil.
- After boiling, the water often tastes flat, and this can usually be remedied by pouring the purified water from pan to pan (they must be clean!) for several minutes to put dissolved air back into the liquid.
- Distillation can often be used to make water potable but volatiles, such as petroleum products will usually distill along with the water. After distillation, the water will taste flat.