No permanent physical damage? What about this from the August, 2002 OLC memo?
...a detainee could suffer spasms of the larynx that would prevent him from breathing even when the application of water is stopped and the detainee is returned to an upright position. In the event of such spasms, a qualified physician would immediately intervene to address the problem, and, if necessary, the intervening physician would perform a tracheotomy.... we are informed that the necessary emergency medical equipment is always present- although not visible to the detainee- during any application of the waterboard.
This is indefensible.