There will be two important factors in the discussion of water. One, where to find a water source? And two, how to disinfect the water after gathering it?
No matter where you are, water will be NECESSARY. We will want to begin by looking at oceans, lakes, ponds and rivers. These will be the most common places to start. However, if you are not in a location you will need to be able to travel a distance, or make use of rain water.
After collecting water from your source, it is VERY important to disinfect it, otherwise you and others can get very sick. The most common method of disinfecting is to boil your water. However, this is not always feasible as it takes energy in order to heat and boil the water. If you do not have enough electricity or wood for a fire, you will have to focus on other methods. SODIS is a very good example. This stands for Solar Disinfection. By placing your water in plastic see through containers, and allowing the sun to hit the bottles, this will kill bacteria and viruses in the water. Another point to consider, what if there is no sun around, for weeks? You will want chlorine tablets.
More will be discussed as time goes on.