Using "water".

Personally I prefer to buy 'a water' in Topless Bars.

What are topless bars?

Bars where there are no tops on the bottled water.

;-)))

I've noticed that in Europe, if you ask for water at any eating establishment, you will usually be served a bottled water -- and charged for it -- whereas in the US, restaurant water comes out of the tap and is invariably free, unless you specifically ask for bottled water (and then I don't know how many restaurants keep that on hand). So it depends on where you are.

One thing funny in England, Europe, and Scotland: they always ask "gas or no gas?" or "sparkling or clear" it sounds so funny.

<<"the barman / person"

I think that "bar maid" is Politically Incorrect, perhaps someone would like to comment.>>

Personally, I don't find "barmaid" offensive, but if you'd prefer to keep it gender-neutral, just say "bartender", or, if they're the bartender's assistant, "barback".