It really makes a huge difference if you're the prisoner:
Imagine it. You've been arrested. Probably, violence has occurred, and will continue to occur. They're treating you roughly, and with anger.
You're knocked around, strapped to a board, and water is being poured over your face and in your mouth until you can hardly gasp a breath for the choking.
IMO, and certainly this is only an opinion, waterboarding a guy over a hundred times, it's just being done for the enjoyment of causing someone misery, and the idea that a prisoner is going to tell you anything useful after a six-a-day torture session is ridiculous. I'm sure the powers that were made it clear through the chain that they wanted KSM to be absolutely dehumanized.
After twenty times, "Well, he surely must have told us everything he knows."
After fifty times, "Well, surely..."
After eighty... why did they ever stop? How did they finally decide that he wasn't going to say what was wanted?
People react so personally to this because KSM was involved with 9/11... but I point out that more Americans are killed every day because they're too effing fat, or have smoked too long, or drive poorly, or drunk, etc.