I don't know about all LGATs, but in Landmark, you are allowed to take bottled water into the room, (That is the ONLY thing you can have). Also, they have pitchers of drinking water and paper cups at the back of the room, so it is available at all times.
It would be best to take in your own, (if you were going to do it in the first place), because it might be awkward to stand up and go
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Large Group Awareness Training, "Human Potential"