The method of interrogation targeted by the antiwar crowd has been water boarding. Used for hundreds of years under many names, water boarding basically creates the fear of drowning in its indented subject. Images of the Khmer Rouge's use of it were viral across the blogosphere and on TV when it came to light that we were using water boarding, except that modern water boarding used by our intelligence agents is not intended to create pain for pain's sake, as it was before.
The prisoner is bound to an inclined board, feet raised and head slightly below the feet. Cellophane is wrapped over the prisoner's face and water is poured over him. Unavoidably, the gag reflex kicks in and a terrifying fear of drowning leads to almost instant pleas to bring the treatment to a halt. 
At no time is the subject under threat of actual drowning from swallowing water, as it was with old water boarding techniques used by the Khmer Rouge or the Imperial Japanese during World War II. The intent of the method is to activate the instinctive gag reflex to create an uncomfortable situation for the subject. Out of the 600 plus prisoners that have graced the cages of Gitmo, only three have been water boarded, according to the CIA , or a dozen have, according to ex-CIA sources . In no way is it as prevalent as the antiwar-human rights-leftist groups would have you believe. Also, something that may be unknown (unlikely) to these groups, modern water boarding is used on our own soldiers. The Survival, Evasion, Resistance and Escape program puts soldiers under intense, but not life threatening situations, that include water boarding, to strengthen them against enemy interrogations. 
Harsh interrogation has saved many, many lives and stopped many, many plots. Specifically, a plot called the West Coast Plot, featured at National Review, was discovered and disrupted due to the use of harsh interrogation of terrorist Abu Zubaydah.
Zubaydah was captured in a gun battle and severely injured. The CIA arranged medical care, saving his life. After he recovered, Zubaydah provided what he thought was nominal information—including that KSM's alias was "Muktar," something our intelligence community did not know. But he soon ceased all cooperation. It was clear to his interrogators that he had received interrogation resistance training, and the traditional methods were not working. So the CIA employed alternative interrogation techniques. And Zubaydah started talking.
He provided information that led to the capture of Ramzi bin al Shibh—one of the key plotters of the 9/11 attacks and a close associated of KSM. Bin al Shibh was the mastermind behind a plot for a follow-on attack to hijack airplanes in Europe, and fly them into Heathrow airport. Now he was off the street and the Heathrow plot was setback.
Together, bin al Shibh and Zubaydah provided information that led to the capture of KSM.
Once in custody, KSM refused to cooperate, until enhanced interrogation techniques—including waterboarding—were used. Then he began to talk.
He gave us information about another terrorist in CIA custody named Majid Khan. KSM told us that Khan had been tasked to deliver $50,000 to a Southeast Asian terrorist named Zubair—an operative with the terrorist network Jemmah Islamiyah, or JI.
Confronted with this information, Khan confirmed KSM's account and gave us information that led to the capture of Zubair.
Zubair then provided information that led to the capture of a JI terrorist leader named Hambali—KSM's partner in developing the West Coast plot. Their strategy was to used Southeast Asian operatives, since KSM knew we would be on the lookout for Arab men.
Told of Hambali's capture, KSM identified Hambali's brother "Gun Gun" as his successor and provided information that led to his capture.
Hambali's brother then gave us information that led us to a cell of 17 JI operatives that were going to carry out the West Coast plot. 
I have a question for these groups: Are you truly willing to stop all harsh interrogation of captured terrorists because you so doubt the professionalism of our armed forces and covert agents that you believe they'd use a last resort technique on someone who may be innocent even though they have literal tons of pages of information on those within their authority to interrogate? Do you really hold our national security apparatus in such low esteem?
I'm not sure on the specifics of each groups' opinion, but some would say yes.
We are at war. We are at war with a covert, ruthless and very political enemy that does not follow the rules of war set down by the international community and whose goal is a worldwide cultural and religious cleansing based on a perversion of Islam mixed with the insane revolutionary zeal of vanguard parties (right from the pages of Marxist-Leninist theory on gaining power). In war, you do not prosecute the enemy if they are not American. You kill or imprison them until the war is over. Clinton lawyered terrorists and that got us 9/11.
We learned that lesson.