It does depend to some extent on how used to wild water your digestive system is, I think. If you've only ever drunk tapwater or filtered water, I would have thought it would be best to filter or boil or otherwise chuck tablets in. Streams in spate after heavy rain carry a lot more bugs than they would otherwise do and I would always boil water or use sterilising tabs for those times, but otherwise I'm happy to drink most peaty gunge, whether or not the fabled dead sheep in languishing upstream.