Q. At the end of the fast of Tisha Beav that this year will be delayed until Sunday, before one makes Havdala can one drink some soda water when very thirsty, if that is all what is available in Shul?
A. Shulchan Aruch (299: 1) rules that one may drink water before reciting Havdalah at the end of Shabbos.

However, Poskim debate if that permission applies to sparkling soda water that one drinks reciting a brocho, even when not thirsty, because of the pleasantness and good flavor it has. (Shevet Hakehosi 2: 124). Other Poskim are lenient (See Piskei Teshuvos ibid. quoting Shalmas Chaim 273).

Horav Dovid Pam and Horav Aharon Miller Shlit’a maintain that when possible one should be stringent, and just drink tap water.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav
Chanoch Ehrentreu Shlit’a.