I admit that some information may be gained from torture. From what I have been read and heard from military and psychological experts on the subject, it is less effective than other methods at gaining information from prisoners.
Its not less effective than other methods, because they exhausted all the other methods before they waterboarded these guys, which got them the information they needed.
The interrogators have alot of intel to start with, and they ask questions they already know the answers to, and suspect that the prisoner knows. They waterboard him until he finally starts answering those questions truthfully, and thats when they start asking the questions they dont have the answers to.
The arguement by some people is "how do you know if hes telling the truth". They know hes telling the truth because they ask very specific questions about bomb makers and their locations, and they also ask about the locations of high value targets. When they torture a guy until he gives up an address, at that point they send a team to raid the house. If It appears he was lying and theres no terrorist at that location, they come back and waterboard him some more.
Eventually the subject will just want the waterboarding to end and he will give them what they want to know, because on top of being waterboarded, he doesnt eat much, gets no sleep (sometimes up to 9 days without sleep), and they keep him in a cold cell with no blanket or bed. They will entice him with the luxuries of bedding, food and sleep, and after weeks going without, anyone would tell them what they want to know just to make the madness end. I dont know if youve ever gone without sleep for an extended period of time, but it breaks your mind down terribly, especially by about the 3rd or 4th day.
Abu Zubaydah was the first person the CIA used the enhanced interrogation on, that included waterboarding. After weeks of normal interrogation, they grew frustrated because they knew he was hiding information, but he began to adapt to their other techniques, so they argued that they needed to use "harsher techniques", at which point authorization was given to waterboard him.
What information did they get from waterboarding him you ask? He gave up crucial intelligence that allowed the CIA to capture Khalid Shaikh Mohammed (the 911 mastermind), among other intelligence. This was a huge win for the US, in so many ways.
After Khalid Shaikh Mohammeds capture, they eventually waterboarded him too, and they got a ton of information out of him, including him filling in the empty blanks they had about how 911 was planned. On top of that, it gave the CIA a much greater picture of AQs command and control structure.
Before they waterboarded KSM, when they asked him about future atacks in the US, he cryptically answered "you will know soon". After they waterboarded him, he gave up key locations inside the U.S. that AQ was interested in hitting, he gave up the information that stopped a terrorist attack in LA. by an eastern asian terrorist cell, AND the capture of other key AQ allies.
Waterboarding was responsible for saving MANY lives.