OK, so in a comment, Bruce points us to this. An entirely rational discussion: words mean what we commonly accept them to mean. Thus whether waterboarding is torture or not depends upon what we all think the words torture and waterboarding mean.
So in order to decide whether waterboarding is torture or not we need to define our words.
I would posit (and whether this is true or not depends rather on how many agree with this) that torture is the infliction of physical pain in order to elicit information.
No, that\’s not a complete desciption: torture can also describe the infliction of physical pain purely for the pleasure of the torturter, psychological torture is another meaning. But I would put forward the meaning that anything which is the infliction of physical pain in order to elicit information is indeed torture.
Does waterboarding inflict physical pain? Yes. Is it used to elicit information? Yes.
Thus waterboarding is torture.