The assumption that you are making here is that the victim in this case is still operating under normal mental circumstances. In other words, you're assuming that this person is still going to make logical decisions. If that person is being tortured, mentally tortured in this case, this cannot be assumed.
Each time the fear of death is induced, the victim is pushed further and further into a chemical imbalance in the brain causing behavior to be unstable. This instability, as you can imagine, is multiplied by the number of times that person is subjected to waterboarding (the mental torture). The severe mental duress, which you and I both know very well because we're teachers
, lasts not only thru the intervals where the torture takes place, but there is also permanent damage done to the human psyche and will likely affect the victim's life from now on.