Did you read Dom's site?
I had a thermos half full of water kefir and police officers did an alcohol test on it with an electrical device and it passed as non-alcoholic, and I let mine brew until all sugar is gone.
Kids like it and it's so much better than soda.
It's most likely going to vary quite a bit based off many factors like how long you ferment, how long its stored, how much sugar you put in. I believe water kefir on average has a little more alcohol mostly due to the amount of sugar and a greater ferment time (48 hours standard for water vs 24 for milk).
Yemoos has an FAQ on water kefir and here's what they say about alcohol content
Does kefir contain alcohol?
Yes, its been found in a couple studies now to contain about 0.038% - 2% alcohol, or 16-38 g/L
(grams per litre). With the normal amount being around .08 or less (for a 48-hour ferment). Kefir that
is stored and ripened for a few of days will continue to increase in alcohol, up to 2-3% (when it is
sealed tightly). For reference, beer contains about 4-7% and wine 8-14%. Because kefir contains
bacteria (and not just yeast like beer or wine) the amount of alcohol kefir can produce is limited by
the acetic bacterium which convert the alcohol (produced by the yeasts) to beneficial acids.