In a real survival situation with a source of fresh water but no purification gear, you'd just drink it and hope to not get sick. Chances of actually getting sick aren't that high. Since you're doing a simulation, I think you can bring some pills along and pretend you were using the water untreated. It's just one variable you're removing.

I knew of a guy in Boston who was jilted by a romantic partner. He was extremely upset and depressed, but he couldn't make up his mind whether to commit suicide or not. So he decided to drink a glass of (disgusting, polluted) water from the Charles River and see what happened. He did so, and waited. Nothing happened, and he is still around. I don't know if he's any smarter than before. Anyway, untreated water isn't necessarily fatal.