I believe what was meant by 'drink plenty of water' while taking kefir was that one should drink plenty of water throughout the day - not when the kefir is taken.
I think that taking it at night a couple of hours after eating is better because I have heard that the probiotics in the kefir have a chance to colonize. (All night - verses when taken in the morning although the stomach acid is low they don't have much time for colonizing before eating.)
I drink about a 1/2 cup of water after taking the kefir.