Technically it is supposed to be distilled, no shop I have worked at ever (nor the dealer) uses anything but tap water. Unless you have hard water there just use tap water.
The caveat to this is you don't want to dilute the coolant too much or you'll lose the anti-corrosion properties. So if it keeps happening then get VW/Audi coolant.
And the system holds like 2 gallons, so at the bottom of the bottle isn't pretty low, that's only like a quart
There are no Audi/VW dealers on island. However, there is a BMW dealer that sells "BMW coolant." Is it safe to assume this is the same or equivalent as the recommended G12?