Despite the administration denials:

In the NY Daily News, an opinion column by Malcolm Nance: I know waterboarding is torture - because I did it myself.

Having been subjected to this technique, I can say: It is risky but not entirely dangerous when applied in training for a very short period. However, when performed on an unsuspecting prisoner, waterboarding is a torture technique - without a doubt.
...In the media, waterboarding is called "simulated drowning," but that's a misnomer. It does not simulate drowning, as the lungs are actually filling with water. There is no way to simulate that. The victim is drowning.
Unless you have been strapped down to the board, have endured the agonizing feeling of the water overpowering your gag reflex, and then feel your throat open and allow pint after pint of water to involuntarily fill your lungs, you will not know the meaning of the word.