I think anyone who goes into a bar or restaurant and orders a glass of tap water and nothing else deserves to be charged for it for being a total tightwad.

Think about it - you take up space at the bar or at a table, the waitress has to fetch your water then your glass has to be washed in the same manner as if you had drunk a beer or a soft drink.

Ordering a couple of other drinks and/or a meal should mean that your tap water comes free of charge and, in my experience, it does.

Having said all that (and I think I already posted this on a similar thread about tap water) I was once given a (free) glass of tap water in a restaurant after coming over all queer when I was heavily pregnant. So, if you want free water, either get pregnant or make use of one of the zillion water fountains around the city.