One source describes --
"Ordeal by Cold Water: The prisoner was strapped down so that she could not move a fraction of an inch. Water was allowed to fall, drop by drop, on a selected part of her body -- the vessel from which it dropped being in full view to the sufferer so she could see each drop coming. The most deadly torture by this means was with the forehead as the point of impact. Applied over a long or even short period, it eventually drove the prisoner out of her mind. Worse than this method was the continuous stream variation. In this, the flow of water fell from about six feet on to the forehead of the victim, quickly reducing her to a state of absolute surrender within seconds."

Brian Innes's The History of Torture also mentions that:
"An alternative was to pour a continuous stream of water from a height on to the victim's forehead."

. . . but now, read what is really happening in this picture:

We discover that this "treatment" is a yoga technique called shirodhara, said to relieve stress!

I can't believe there would be that much of a difference in the effect between cold water and warm oil. Self-experimentation with the water torture has proven to be inconclusive -- although I must admit I was always able to end the torture/treatment at any time!