I don't have any idea what yeast-fermented soda is, sounds like it might be worth a try.
I do make water Kefir and I really do enjoy it and it does have some health benefits, but nobody is sure quite what they are other than digestive. I am with you on the milk, yuk.. The beauty of water kefir is once you get your grains growing which is a natural process of making the kefir you will have a lifetime supply and you won't know what to do with all of it. I usually give a couple bottles away from each batch and if someone seems to like it then I offer them a chance to make their own. Right now I am making orange and lemon kefir because oranges were on sale. I also really like strawberry/lemon more or less a lemonade, very refreshing. I can give you a really good source for grains, not all sellers are as good as this one.
I just noticed his prices went up, But they are excellent grains that grow out very well and you will never have to buy them again.
http://www.amazon.com/Keysands-Organic-Grains-Crystals-Tibicos/dp/B0051WV5K6/ref=sr_1_2?ie=UTF8&qid=1310389434&sr=8-2What I do know is that grains grow much better in the warmer months. You can save off some as a just in case and store them in the fridge. It is a great summer time drink with some health benefits and it is really cheap to make, you don't have to flavor it but I just look out for fruit that is on sale, pretty much anything will work.
What I have found out is if you put the fruit in while making the kefir it does make the grains dirty so I make the Kefir and then flavor it. If it is something like orange, there is plenty of juice to puree everything, for other things I use a little kefir for the base liquid needed by blenders. I just use my immersion blender in a big bowl and put it into a plastic bottle(we use a lot of bottled water so it is easy for us, not sure how to do it another way but you will find out for your self.
It is so easy to make, best to start with 4 tablespoons of grains but most sellers only give you from 1/2 to 1 tablespoon, which is why I like Keysand he gives you a full 4 tablespoons, actually a bit more.
Use this basic recipe, and then just multiply it by considering how much grain you have. Just making the kefir this time of the year, those 4 tbs will grow out to 6 fairly quickly, then you can make more kefir. Some metals will react to the grains and kill them, so it is best to not take a chance and use all plastic utensils.
1 tablespoon grains(rinse before each use, warm but not hot water, hot water will kill your grains)
1 tablespoon sugar(household sugar is fine but anything including real maple syrup can be used)
1 cup of water
So with 4 tbs of grains you can make 4 cups of kefir. Cover and let set in a warm place for 48 hours and what will be left is something that is not sweet because the grains used the sugar, could be slightly tart and may have some bubbles, the process does make a slight amount of C02.
Now you know what I know about Kefir, good luck.
Barry