Reach Chapter Seven in the single player campaign to unlock the Campfire and upgraded Hanging Pot. Build both for a renewable source of clean water. Construct a canteen to keep a large source of clean water. You can also find clean water in bottles around the environment. Search every area to get enough to survive until Chapter Seven. If you build a canteen early and fill it with infected water, you will have to drink it all to clear space and replace it with clean water.