4 1/2 liters of water per person per day is recommended to maintain hydration during moderate activity. This means you will need a reliable source of water and then know how to treat it to kill all sorts of viruses and diseases that may be in your water source.
Possible sources of water can be obvious like a stream, lake, or pond. Others may be a bit more difficult to acquire like rainwater or a solar still. If you can only find a small pond you need to inspect it as sometimes animals die while drinking and fall in, this should be avoided due to obvious reasons. Look for signs like animal tracks or paths. Animals have to drink too and their paths will usually go to a drinking source.
Once you find your water source you will first need to treat the water, this is the easiest step that most people are misinformed about. With a water temperature of 160 to 165 degrees F it takes just half an hour for all disease causing organisms to be inactive. At 185 degrees F this is cut to just a few minutes. By the time water hits its boiling point of 212 degrees F and then cools the water is safe. This applies even at high altitudes.