Block, Dodge, Parry- dumb question

Just got the OSR pdf. Plan on running my first ever campaign revolving around goblins who brewed up a monsterification plague. Open to advice for first time wardens but my dumb question is about the waterskin. Can it be filled anywhere and is there any mechanic or suggested mechanic for untreated water? I'm not sure if just any water source is enough to refill endlessly or if you can only get one full d8 usage die for it a day or something. Any clarification is much appreciated.