Coconut water has sugar in it. The kefir feeds off the sugar in the coconut water. It is just a matter of knowing how much sugar is left when you consume it.
I make my own water kefir. I leave it to fermen for 2 days, then I strain it and put it in the fridge for 2 more days. By the time I drink it, it is very sour, and contains little sugar.
If your kefir tastes too sweet, just leave it longer. This way you can control the sugar content.
If the cocnu water kefir tastes sweet, it will have sugar left in it too.