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, a varying small amount dribbling into it every few Roundworld minutes or so.
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.
Blue water can be mixed with healing tea, possibly combining the healing of both liquids.
When drank, blue water will immediately restore some of the drinker's HP, specifically 2.5 HP per drop of blue water. Unlike healing tea, blue water does not have any effect on the drinker's regeneration rate.