Primo Levi in his book "If This Is A Man" aka "Survival in Auschwitz" specifically states that those destined for murder in Birkenau were not provided with water.
Was this consistently the case? Was water deprivation part of the standard operational procedure for the SS?
In the twisted world of the Destruction Camps, it makes sense. Thirsty people will eagerly flock towards a sign that says "Showers"; and when the thirsty Primo Levi was in the actual shower of Monowitz-Buna he describes it as "minutes of bliss" presumably because he was finally able to drink after days without water.
Edited to add:
In his autobiographical book "The Pianist", Władysław Szpilman describes how the water supply to the Assembly Area was deliberately cut off by the SS, and he describes children dying of thirst as they went from person to person begging for water.
Women carrying children dragge themselves from group to group, begging for a drop of water. The Germans had turned off the water supply to the Umschlagplatz on purpose. The children's eyes were lifeless, their lids already drooping over them...their dry lips were open like the mouths of small fish discarded on the bank by the fishermen.
I am driven to surmise that this went beyond wanton cruelty -- thirsty people crowd towards "Showers" if only to drink one last time prior to death. The policy may have become SOP for the SS, initially inspired by viciousness, but then implemented when it was discovered that it simplified the mass murder process.
I suppose that extant records of the SS procedure are not available?
There are multiple accounts of minimal water at best being supplied at the start of the journey, which was quickly exhausted. Many of those who survived the cattle cars have spoken of how there was no food or water, and no sanitation - there are several accounts of people desperately licking at the water sprayed into the cars when the stench became offensive to the soldiers in charge of the trains.
The mentality of it is that the Jews were considered less than human. Logic aside, there was no desire to provide food and water and sanitation, to the SS mindset these were not human beings. They were animals being herded off to be culled or enslaved, nothing more.