One of the first questions that I always here from people visiting Poland (after "what is the internet password?") is "can I drink the tap water here?". I completly understand because that would be one of mine first questions in the foreign country. And the anwear is: Yes, you can drink tap water in Cracow and anywhere else in Poland. I do it and I completyl don't understand how people can spend money on something that is for free and not in a plastic bottle.
But you will see a lot of poeple buying 20 or more bottles of water, so you might think it is not very safe to drink the tap one. The thing is that it's just not very popular. Somehow there is this myth about botlled water being better. I used to believed in that but I drink the tap water for so many years now and I am fine. Water taste like water to me. But because it's not so popular you won't get the water from the tap in most of the restaurants. I wish I can just get a jug of water for free before dinning but instead of that I can get a little bottle of water for double price. It's ridicouls, but this is how it is in more restaurants. Only if you see the sign that is in the main picture, means that they serve water from the tap.
So don't worry! Open the tap and drink it. I think it's smarter to spend your money on polish craft beers than for the water which will probably taste the same as the water from where you come from.