No. You cannot safely drink the water from the river on the Narrows hike in Zion. The Virgin River experiences blooms of cyanobacteria and cyanotoxins— really dangerous toxins in novice terms. Zion will always keep an updated list of river sections impacted. The naked eye can usually see these toxins as green foam, but the water that looks clean could also be contaminated. Don't eyeball it. Trust the park. No amount of boiling, treatment, or filtration will kill these toxins.