The water in Norway taste so good and are safe to drink. Don't spend your money on a bottle of water, its really expensive and totally unnecessary.
When at a restaurant just ask your waiter for tapped water. When in Oslo she will bring you water from Mariadalsvannet - it will be clean and feel crisp.
My family has a cabin in Telemark. Its been in the family for more than two hundreds years. Everyone who's lived on the farm turned over hundred. We believe its because of the water. The well that supplies us with water comes from the mountains and it must contain eternity...Its just magic:)
So spend your money wisely, instead of bottled water spoil your self with a "skolebrød" - a typical Norwegian bun.
Check out the recipe at homemade by Restaurantdronningen