Water kefir has probiotic microbes in it that is beneficial for gut health. The health of your gut bacteria is critical for overall good health. A diet high in sugar can kill the good bacteria and let the bad bacteria grow uncontrolled leading to a host of chronic problems linked to diabetes, high blood pressure, and more. For studies and more details, see here: http://newsy.tiddlyspot.com/#Microbiota%20main
Other things that kill good bacteria in the gut are: artificial sweeteners like aspartame (sold as NutraSweet, Equal, and Canderel) and sucralose (sold as Splenda, Zerocal, Sukrana, SucraPlus, Candys, Cukren, and Nevella), excessive alcohol intake, frequent sugar intake. As far as saccharine goes I consider it safe in small doses. The rats in the study with saccharine were given 100x the normal intake of saccharine and that’s why they had problems. There is no evidence saccharine causes problems using a normal dose or with occasional use. Stevia is generally considered safe (I use is most days) and you can read this study: http://newsy.tiddlyspot.com/#Stevia%20and%20microbiota
You can make your own kefir from dry granules but it might take 24 hours in the water for the bacteria to become clinically effective. It will have live bacteria after an hour, but that is not the same as being “clinically effective”. The bacteria has to grow to a certain population to actually give the body benefits. But getting a moist mother is the best idea as long as it is not subjected to freezing or very hot temperatures during shipping.