In Vienna we found that we couldn't get tap water in restaurants and had to pay around 5 euro a litre. Make sure you specify "still" unless you want to get bubbles in you water. For touring we just filled our own water bottles in the hotel from the tap and drank it when not dinning and had no problems.

In Salzburg they would serve tap water without any questions. In some restaurants they even brought it before the meal. No charge. We liked Salzburg