If you're truly stuck in a pinch and need clean drinking water, there are ways to filter it to make the water safer. The water won't be 100% pure after using a 2L Bottle Water Filter but it's better than nothing.
You should always boil your water regardless.
Golf Ball sized Rocks
Cut about 2 inches from the bottom of the bottle. Make sure the cap is screwed on but not too tight.
Stuff cotton into the cap area. You can use whichever type of cotton you have. Cotton balls, squares, fluff. This is the very bottom and last thing the water will be filtered through.
Add a coffee filter. You're going to put a coffee filter in between every layer.
Add a layer of "activated charcoal". Remember, you can easily make your own wood charcoal.
Add a layer of sand and or clay gravel. You can buy pure clay cat litter at the dollar tree.
Now, add a layer of green, healthy grass. The grass contains chlorophyll which has antibacterial properties. It helps if you cut it up with scissors to activate the chlorophyll and expose it to the water.
Next, add a layer of small pebbles and then a layer of medium sized (or golf ball sized) rocks. In theory, the rocks will help break any bacterial walls following the same principal behind what happens in a flowing river.
Now, unscrew the cap and run the water through the filter. The water won't be 100% pure but it sure beats the alternative. Remember to boil your water regardless. A filter only helps remove impurities. Boiling will actually kill most of what lurks in your water.