There are a number of water fountains located throughout the airport, see below for the locations. Alternatively you can request tap water at any food/catering outlets.
Before Security
Opposite the Spar store, on the Ground Floor
After Security
Within the main departure lounge, near to our Assisted Travel Desk
At Gates 1-20, to the right of the entrance
International Pier Bussing lounge (Gate 49), near the toilets
Arrivals Area
North Baggage Reclaim area