The Mezzanine

But if I ask for hot chocolate or milo or juice and they don't have it then it puts them in the position of having to refuse my first/second/third choice of drink and therefore unwittingly being a bad host; and putting them in that position makes me a bad guest. Whereas if I ask for water then they can grant my request easily, thus fulfilling their hostly duties to give me my first choice of drink, and everyone's happy.

Trust me, I've tried 'just' saying "I don't like tea or coffee", and I've got everything from confusion through to panic. And I've occasionally tried asking tentatively for milo or whatever seems most likely, and if they don't have it (they usually don't have it) they get super apologetic and I have to spend a long time saying it's okay and they still feel guilty and I get irritated at having to say it's okay *again*. Whereas if I just say "A glass of water would be great thanks" then they can just pour me a glass of water exactly the same as they'd pour someone a tea.

And certainly if I'm desperately wanting to go I'll make excuses and go if I politely can. But sometimes I can't politely because my host is the kind of person who Just Keeps Talking and you can't get a word in edgewise without practically saying "I don't care about the end of your story, I have to go". In these situations the only resort is to pointedly finish drinking all my water and angle my feet towards the door and fiddle awkwardly with things in the hopes that they'll pick up on my body language and wrap up their story.

Of course I'm exaggerating for effect, and anyway not everyone with an empty water glass is that desperate. And *anyway* my original point wasn't even about the guest's comfort except in passing; I was responding to your expressed discomfort with the odd-lookingness of guests fiddling with empty water glasses when the conversation lags, and suggesting that if you as host are uncomfortable then you can just as easily create a gracious exit too.

tl;dr I'm just trying to refute the idea that when a guest requests a glass of water it's "the worst". Unless of course you live in a place where clean water is legitimately not easy to lay your hands on. But otherwise if they ask for a glass of water, give them a glass of water and don't stress about it - or even put it in a mug if you don't want to have to worry about the odd-ness of the empty glass.