Water and lots of it!
Somebody mentioned Kevita, which is a great choice and is one of my favorites. Especially the one with only 1g of sugar.
I'd say if you have a taste for it any low sugar kombucha is great. It's refreshing and I often make it look like a "drink" with ice and wedge of lime.
This works really well, when I am at party where I don't feel like explaining my sober lifestyle.
(Sometimes I do the same with club soda and a splash of cranberry)