I sometimes ask for water when travelling ET, which admittedly isn't that often, but I did so twice last week and was served immediately. It was only because I didn't have a mobile phone with me, I've no problem paying since I mentally add £2 to the fare. In both cases they had some not-totally-full bottles on the trolley, so it may not have come from the tank. I can't recall being forgotten, I'm more likely to be offered a free mini bottle by the SCCM.

Tea and coffee is made from tank water and of course that isn't taken anywhere near 100c along the way. So far it hasn't killed me.