This is normal at virtually all sports and music events and festivals i visited in the Netherlands. If you're lucky you can bring in an empty bottle and fill it at one of the two tap points that all other 50.000 attendees are using too. If you're less lucky you have to throw your bottle away and pay for your water the same price as your beer (usually two euros or so). If you're even less lucky they don't have any water at all and you have to either get drunk or drink sodas.