Human Rights Watch has explained:

Waterboarding is torture. It causes severe physical suffering in the form of reflexive choking, gagging, and the feeling of suffocation. It may cause severe pain in some cases. If uninterrupted, waterboarding will cause death by suffocation. It is also foreseeable that waterboarding, by producing an experience of drowning, will cause severe mental pain and suffering. The technique is a form of mock execution by suffocation with water. The process incapacitates the victim from drawing breath, and causes panic, distress, and terror of imminent death. Many victims of waterboarding suffer prolonged mental harm for years and even decades afterward.

As for the efficacy of the torture -- a proposition that is dubious at best -- the question always is: At what cost? Is it worth it to get that data at the price of becoming the world's greatest monster?

As the late Joan Fitzpatrick put it:

The prohibition on torture is a peremptory norm of customary international law binding on all nations. The torturer is the enemy of all mankind.

And it will always be so. No matter how much fearful little worms like Joe Lieberman want you to forget it.