As anyone knows, you are going to need to find a source of clean water in order to ensure your long term survival. The length of time that a human can live with out water depends on various conditions such as, level of exertion, temperature and types of food that you are eating.
If you are eating moist foods such as celery, you can survive longer. There are several different animals that survive and thrive in the wild and never actually drink any water. Us mammals however, are 70% water and require it to stay healthy and alive.
A stream or well are the most obvious sources of clean water, but you should also know what is upstream or if there are any contaminates in the well that might be harmful to you. Contaminates could include things such as a farm that uses a lot of pesticides, a dump or landfill, radiation that has fallen to the ground or even a zombie that is trapped down there.
NEVER DRINK STAGNANT WATER! This means water that is not moving, like a swamp or ditch. This kind of water can have harmful bacteria and all sorts of tiny parasites that are going to make you extremely sick. You could get massive diarrhea which leads to dehydration and eventually death if you don’t get better quickly. If you had to pick a way to die, crapping yourself to death would definitely be last on that list.
There are many sources of water that you can find in urban environment too. These include the pipes inside of houses and water heaters that are full of hundreds of liters of clean drinking water. Just make sure that the house that you are scavenging from is abandoned because in a survival situation, people are going to be very protective of what ever few resources that they have left.
If all you can find is undrinkable water then there is still hope. You can clean and sterilize it in order to make it safe for human consumption. The quickest and easiest way is to filter it through several layers of cloth. This will remove a lot of particles that might lead to gastrointestinal distress. Most of the time filtering your water is not enough, so you are going to want to build a fire and bring your water to a boil for at least 5 minutes to make it completely safe to drink.
If the water that you found is salty or chlorinated then those two methods that I previously described are not going to be enough to make the water drinkable. In those situations, your best bet is to create a solar still to remove the clean water from the contaminates. You can also use a solar still to make clean drinking water from your own urine. I know that you have probably seen or heard of people drinking their own urine to stay alive but this is only a good technique for the first pee. If you keep drinking your urine after that, you are going to poison yourself. Only drink urine as a last resort. It is waste and should be treated as such.
Those are just the basic ways that you can find and clean your own water to ensure your long term survival. There are a lot more in depth techniques that I have not described here are if you are creative, you might even be able to make up some of your own. So good luck out there and hopefully you will never need to use this information.