During wars and tourneys, fighters often risk dehydration. To prevent this, people bring water to the fighters. These people are often the fighter's consort, or of the same household or group as the fighters. There have been many ingenious ways developed of getting water to weary fighters. Some carry large water barrels carried on their backs with plastic tubing for straws, others carry a box with compartments filled with smaller waterbottles, each bearing the name of the fighter who the bottle belongs to.
Your friendly neighbourhood Chirugeon reminds you to have your own water bottle, to avoid sharing nasty diseases.
Water bearing in period
Here's an example from the battle of Dorylaeum, during the First Crusade.
Feminae quoque nostrae in illa die fuerunt nobis in maxilo refugio, quae afferebant ad bibendum aquam nostris peliatoribus, et fortiter semper confortabant illos, pugnantes et defendentes. - Liber III, Gesta Francorum et aliorum Hierosolimitanorum
Roughly: "The women were of great assistance that day, for they brought water for the warriors to drink, and bravely encouraged the fighters and defenders."