Blue water is a liquid which heals the drinker.
Blue water can be found in the fountain at the centre of the atrium inside the Temple of Small Gods in Ankh-Morpork. The fountain replenishes itself slowly at around two tablespoons per two roundworld minutes.
A player can drink from the fountain directly, but the common approach is to fill a portable liquids container such as a bottle or canteen so they can heal themselves in any location.
When drank, blue water will immediately boost the HP regeneration of the drinker. Unlike healing tea, blue water does not have any effect on the drinker's regeneration rate.