I would have no problems taking a couple of little bottles of water from the Lounge (or a packet of biscuits). Generally if you stay in a good hotel, the concierge lounge or reception or whatever will be giving you these things anyway to take with you (and an airport lounge seems pretty similar to a concierge lounge to me - there to make travelling easier and comfortable).

I don't think that anyone is going to mind, providing you are sensible.

Boo