Last year I read a certain book that extolled the virtues of fermented coconut water, but gave no instructions for making it. Instead, the author's website sold the drink and it wasn't cheap. I wanted to try it myself, so I went on Ebay and bought water kefir grains for about $6.
Now water kefir is kind of like dairy kefir, but it supposedly thrives in just sugar water rather than milk. The problem was that my grains never really thrived. There is a host of conflicting and bizarre information out there about how to treat them and somewhere along the way I did something wrong. I tried all kinds of different fancy sugars, spring water, artisan dried fruit....but they never reproduced. Whatever, I still got benefits from them even if they are pain to take care of. Since you don't ferment kefir as long as kombucha, cleaning and feeding them was a chore I had to do every other day. Maybe they were unhappy because I went on vacation and left them in the fridge....it's hard to find babysitters for tiny gelatinous bacterial and yeast colonies. However, I plan on buying more soon and hopefully I can figure it out, because I really enjoyed the drinks I made.
I suspect the reason that people buy expensive coconut water is that the way to make it is NOT to put your kefir grains in the coconut water. You should do a normal water kefir ferment that consists of sugar and lemon for a day or so. Then use that fermented water and mix it with coconut water or whatever juice you want in a nice bottle with a good stopper. A couple of days later you should have carbonated fermented coconut water. It's probiotic and has less sugar than normal coconut water. Ferments of other fruit juices are delicious too and water kefir is less harshly acidic than kombucha.