I make my own kombucha, it is great. Although I am no a diabetic I do test my blood sugar to see what effect different foods foods have. Kombucha has no great effect on my blood sugar.

I was not even considering water kefir an option for me because of the sugar but after investigation I am convinced that the majority of the sugar is processed by the culture with about 20% being left behind. I calculate that at 10g per liter after fermentation and probably less since I let mine ferment longer than most. It does not taste sweet in the slightest (If it does it needs more time in fermentation) I drink a few ounces a day so very minimal. I really enjoy it and think it's worth consideration especially by those who are off dairy.

It isn't the same thing, but I happily drink kombuncha which also has sugar in it. I just let mine go a little longer.

It doesn't hit me like sugar, and I don't get the sugar lift and crash, but it might effect others differently.

I've been looking into making my own water kefir, too. From the info that's out there, the amount of sugar left would be very minimal because the bacteria feeds on it to produce the fermentation. Obviously you probably shouldn't go crazy adding in loads of dried fruit into your recipe as some recommend, but the benefits outweigh the teensy bit of sugar left in my opinion.

Try different fermentation times to see what's sweet and what's not. Then go for the less sweet one if you're worried. Good luck!