These days, it’s not unusual to carry a LifeStraw or other lightweight filtration method (like this one from LifeSaver) – but if you find yourself in a survival situation without your kit, good drinking water becomes no less important. In the absence of of water purification tablets or filtration systems, you can boil your water until it’s (more or less) safe to drink.
Try to draw your water from a moving source (no stagnant water) and make sure to remove any debris from it. Bring the water to a rolling boil in a metal pot and keep it there for at least 60 seconds (except for at altitudes about 2,000 meters, then boil for 3 minutes).
Let the water cool, and have a drink — it’s as simple as that.