A lot of people put a few drops of dish washing soap in the water. I have a spray bottle I use that stays on the bench that has a few drops of soap in it. If I am tooling I will get the leather wet and then store it overnight in a plastic bag. Before I work on it I take it out and make sure it is not too wet. If stamping I will spray it let it set for a few minutes then start, if it starts to dry out i spray a little more on it. People say that the soap in the water helps the tools and knife slide easier. Mineral content in water can stain leather but I think it would have to be pretty extreme.
David