If you go to the bar and get soda water from the bar tender,, it may not cost anything but a tip is expected since there is no tab or bill that automatically calculates a tip.
When I get soda water in a real glass at the bar and if they dont charge me,, I normally give the bartender a buck.
If you order a "water drink" and dont tip,, its not being very cooth.
So, the answer is,, it's probably free but a gratuity would be in order since the server has to go through the same motions as if you are getting a $1.50 pop/soda,, he is just pressing another button on the spicket.
If you get a "free" water drink and dont tip,, I guaranty you it WILL not be forgotten,, so follow common courtesy rules and things will work out fine.
