What is the ruling if I am asked about the ritual purity of water: should I answer the questioner with its ruling (pure or impure), or should I tell him not to ask about the ritual purity of water because the default status of water is purity?

If you know the water is impure, then inform the questioner about it, as this is part of the obligatory advice due to a Muslim. However, if you do not know of its impurity, then it is not incumbent upon someone who wishes to use it to inquire about it, for the default state of water is purity until proven otherwise.