H2O is the third priority in an immediate-survival-situation. According to the Rule of 3 most people only last about 3 days, although there have been records of people surviving a little over a week in extreme conditions without hydration.
The human body is made up of about 75% water. The Earth is covered 75% by water, although it is quite salty. Water is essential to life on our planet, and in places with no water at all does life not exist – at least as we understand life to be. In many survival situations obtaining potable (drinkable) water is not hard to do. The problem arises from the life-giving and sustaining properties of water. Many bodies of water are loaded with harmful bacteria and parasites so the direct drinking of it can lead to contamination, infection, or infestation, and thus becoming sick. These sorts of contamination can lead to vomiting and diarrhea which causes the body to loose vast amounts of water very quickly, leading to dehydration and death -the irony of trying to get or stay hydrated if one isn’t careful.
This is where fire comes in. In the Sacred Order after the need of shelter has been met, and fire – the tool of tools, has been procured, fire can be used to heat water to kill and, thus, disinfect making it safe to drink. In fact, Water temperatures above 160 degrees Fahrenheit kill virtually all pathogens within 30 minutes, including bacteria and the two prime backcountry nemeses: giardia and cryptosporidia. At 185 degrees Fahrenheit, they're dead in just a few minutes. Almost nothing can survive sea-level boiling temperature (212° F) for any length of time, though a few pathogens like botulism can persist at even higher temperatures .
One thing to keep in mind though, is that some bodies of water may be so chemically contaminated with salt, alkaline, or other chemicals, natural or man-made that it is not drinkable at all even if boiled