It's advised to drink plenty of fluids whenever hiking or on an outdoor trip. Making sure you're hydrated is essential to enjoying your trip!The streams you find are generally safe to drink. Take a look at the rocks in the riverbed, if they are colored more strongly red/orange/yellow than the riverbank there are probably some sediments in the water so it might taste a little weird. Still, you can generally assume that the water is safe to drink.There is plenty of water around, except on a 5-6 km stretch just north of Þórsmörk. Most people will get away with carrying a 1 L bottle and refilling it whenever possible.