1. It compromises the safety of our personnel in enemy hands
2. It is torture
3. Torture is useless for extracting information
The first and second arguments are easiest to dismiss.
In 1985, former Marine and DEA agent Enrique Camarena went undercover in Mexico in pursuit of Mexican drug lords. He was identified and tortured for two days straight on the probable orders of Mexico's Secretary of Defense, General Juan Arevalo and a doctor was used to bring him back from the brink of death time and time again before he was finally killed. Among the implements used were a tire iron, red hot metal pipes and a cattle prod. By the time his body was found, most of his head had been crushed.
Also in 1985, a former decorated Korean War veteran and the CIA Station Chief in Beirut, William Francis Buckley was kidnapped by Hezbollah while working on a plan to rescue the hostages. His torture was videotaped and the videotape was sent to the US Embassy. Buckley was tortured for fifteen months and only his remains were left behind to be buried.
In 1985, the United States respected the full legal rights of drug dealers and Islamic terrorists. It assigned them lawyers and treated them as any other criminal suspects. That did not help Camarena or Buckley in the least, the very same sort of groups who use real torture do not care about reciprocity and that is why there is no sensible reason to treat them with kid gloves in the hopes that they will treat our captured personnel likewise. We know they won't.
United States prisoners have been tortured or assaulted in virtually every war in the 20th century from WWII to Korea to Vietnam to Iraq to Afghanistan. The fact that the United States respected the rights of Nazi, Imperial Japanese, Communist and Islamist prisoners did not in any way restrain the followers of these totalitarian ideologies from abusing our captured soldiers. If anything it made it safer for them to do so knowing that there would be no retaliation.
The cases of Buckley and Campena are reminders of what real torture is. Waterboarding is not torture. Torture is a form of physical brutality that causes enduring physical damage. Waterboarding is psychologically, not physically, damaging. It does not leave permanent scars, it does not maim and does not kill. The potency of waterboarding is in its ability to break down psychological barriers, much as sleep deprivation, extended interrogations, hooding, intimidation, solitary confinement and other forms of psychological pressure do. When waterboarding opponents conflate physical and psychological pressure, they create a dangerous moral equivalence between the murderers of Buckley and Campena and those who are trying to bring them and their like to justice.
The third objection regarding the integrity of information obtained under the influence of such tactics is the most relevant. Indeed information directly obtained through torture is of dubious value. However, the purpose of interrogation is to first break down mental resistance and then extract information. There is no one who can seriously argue that waterboarding is not an excellent tool for achieving the former. Contrary to the impression produced by TV shows such as 24 in which pain leads to immediate confession, the purpose of physical and psychological pressure is to break down the psychological defenses of the detainee in order to bring him to a state where he is prepared to provide truthful answers. That is why psychological pressure is more effective than physical pressure, attacking the source of the mental resistance, rather than sadistically savaging the body.
Opponents of waterboarding claim that uninterrupted waterboarding would cause death. Of course so would any activity that went on uninterrupted. Handcuffing a prisoner for a long enough period enough could cause death as well. And has. These same opponents typically focus on how emotionally traumatic waterboarding is. Which is, of course the point.
Some forms of psychological pressure are used by law enforcement on a day to day basis. There are people who experience psychological trauma after an encounter with law enforcement. But if law enforcement personnel were not intimidating, they would be useless. In preparation for the 1972 Munich Olympics, police were equipped with candy and flowers to distribute in response to any trouble. The result was the infamous Munich Olympics Massacre because peace and love is simply not a useful deterrent.
To accomplish any goal, pressure must be applied and motion cannot exist unless energy is applied. In contrast to the methods used by terrorists and drug lords, waterboarding is a humane way of applying minimum pressure for maximum results. It is not a pleasant experience but it is a good deal more pleasant than the fatal consequences of not applying it. Though liberal opponents of waterboarding continue to insist that we live in a world where coddling prisoners would produce better results than vigorously interrogating them, but evil men fanatically dedicated to their cause will not surrender that cause unless the will that holds them to that cause is broken first.
