Tying them down and pouring warm water over their heads? Nope, doesn't sound like torture to me. Annoying, yes, torture, no.
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Personally, I think that you need to do a bit of research on this before you come to such a snap judgment.
It seems to me that there is a very high probability that at some point, the waterboarded individual's body takes over, and the mind that tells you that this is merelky annoying is no longer in control. Instead, the person has an instinctive drowning reflex, and feels that he will die, and panics.
You can tell yourself as you stick your finger down your throat that there is no reason for you to upchuck and that the finger down your throat is merely an annoyance, but at some point, you get a gag reflex, and BLAAAAAAAGHHH! Up come the cookies, the cobbler, the spaghetti and the Merlot '87.
Waterboarding might well be unendurable torture or it might not, but the only way to decide this is to have yourself waterboarded. Or so it seems to me.
This will be true despite anyone signing any document.