Yes. Treating significant dehydration when it occurs takes precedence over the cleanliness of the water source. Melted snow is safe enough as are mountain water sources without a population center or animals nearby. There are heavily trafficked areas such as the staging area on Denali or the foot of the Khumbu Icefall, where I advise caution. Do not wait for thirst to signal the need to drunk as this mechanism may not work well up high.