The water in Krakow is safe for making tea, cooking , etc. but do not drink it unless it is boiled. This goes for anywhere in Poland. You can buy many different types of bottled water, Polish and foreign, everywhere. It's cheap and good. 

Water processing plants often don't cope with the capacity, cross contamination is possible (it happened last year in several mountain towns!) and it's simply not worth the risk.  Filters need to be regularly cleaned and it is well known that they may breed bacteria.  In the countryside/mountain regions people often have "spring" water, except that it gets contaminated!