It doesn't have to be water.Absolutely any fluid will do(except alcohol of course...and some fluids are obviously more healthy than others).It is always better to clear water infections(or cystitis- same thing but felt differently) without antibiotics if at all possible.They, in this country, are against prescribing too many so the bugs don't get too smart but it is not the best thing, anyway, for individuals to have too many repeated doses as they deplete nutrients in the gut. So it is about drinking more than you actually might feel you want to and spacing that evenly...so that your body never gets to to be dehydrated at any time throughout the day.Sometimes people's period of dehydration arises when they have spent a long time in bed- so even just having a long lie in without having a drink.Good luck with the water infection aspect of your mother's worsening cognition.There is not much else that can be done about the dementia bit though, sadly.