Dehydration increases over time, and can be affected by certain consumables. Much like radiation, it has increasing effects with each increment of 200. It primarily affects Endurance, though Perception, Intelligence, and Agility will also suffer with more severe dehydration. A level of 1000 means instant death. Without outside interference, it will take three days to reach fatal levels of thirst. It is the fastest-acting stat in Hardcore mode, accumulating twice as fast as food and about five times as fast as sleep.
Different foods and beverages have different effects on hydration. Any type of water will reduce dehydration. A drink from a water fountain, or any body of water will increase Hit points, but may also incur radiation damage. Most food has no effect on dehydration/thirst, however, some farmed or wild fruits and vegetables and crafted recipes may reduce dehydration too. Alcoholic beverages, soda and some food may also increase dehydration.
Carrying water—whether it be purified or not—is the easiest way to combat thirst.
The Survival skill enhances the hydration/dehydration amounts of all consumables. A higher Survival skill will make it necessary to carry less water, but also will increase how much consumables, such as alcohol and pork n' beans, will add to your dehydration.
Carrying the Vault 13 canteen is good way to stave off dehydration without having to carry multiple bottles of purified or dirty water.
Water sources, such as lakes and water fountains, will reduce your dehydration by 200 per drink (230 if you have Camel of the Mojave perk).
Strange as it is, the amount your dehydration reduced by using sources of water other than aid items (such as foods items, bottled waters etc.) is not affected by your Survival skill.