Adrian Morris is the safety officer, and was part of the Q&A at our Supporters' AGM tonight, so I asked him what the policy was. He said you are allowed to take bottles of water into the ground.
We get quite a few of our stewards from the uni, and as they're not back yet, we had to use agency...
I take a bottle of water into the north stand every game. I have been questioned at away games, but I have a card which identifies my auto immune disease, and my need to have water with me.