If you were lost in the wilderness and don't have any other method of purifying water, here's a video tip on how you can make a charcoal water purifier out of a plastic bottle. What you have to do is cut off the top of the bottle, then create five layers of material using fresh grass at the bottom of the bottle, followed by sand/dirt, followed by charcoal, followed by sand/dirt and finally more grass. Watch the video for more information.
If you don't have a bottle or other materials for making a charcoal water purifier, check out the following video. According to the person in this video you can dig about 10-15 feet from a water source and have purified drinking water (after waiting about 10-15 minutes for the sediment to settle in the water).