First, the question of whether or not waterboarding (or other "enhanced interrogation" techniques) constitute torture. Torture is typically associated with actual injuries, particularly physical or psychological trauma which will persist after the initial interrogation session. Given that the late Christopher Hitchens was waterboarded for Vanity Fair (article, video), Mike Guy was waterboarded for Playboy (article, video), and soldiers posted to Afghanistan have even been known to waterboard one another while bored (video), I find this somewhat questionable. Waterboarding is clearly very uncomfortable, and causes extreme short term distress. However, I question whether that equates to "torture" in anything but the most broad definition.
The second objection is that waterboarding, "enhanced interrogation", and torture isn't effective at coaxing information from detainees. On this point, I want to defer to Marc Thiessen, author of Courting Disaster: How the CIA Kept America Safe and How Barack Obama Is Inviting the Next Attack. When speaking at the Heritage Foundation in 2012, Thiessen noted:
"Leon Panetta, the then-CIA Director confirmed, quote, 'Obviously there was some valuable intelligence that was derived through those kinds of interrogations.' Now, if intelligence from CIA interrogations was not critical to the greatest achievement of the Obama administration, don't you think they'd be shouting it from the rooftops? Of course. Panetta's predecessor, Mike Hayden, was more explicit. He said, 'Let the record show, when I was first briefed in 2007 about the brightening prospects for pursuing bin Laden through his courier network, a crucial component of the briefing was information provided by three CIA detainees, all of whom had been subjected to some form of enhanced interrogation.' [...] And he... moreover said, 'It's nearly impossible for me to imagine any operation like the May 2 assault on bin Laden's compound that would have not made substantial use of the trove of information derived from CIA detainees, including those on whom enhanced interrogation techniques have been used,' unquote."During his 2010 remarks at Heritage while promoting his book, Thiessen talked about how limited the use of waterboarding had actually been, how effective it had been at eliciting information from top al Qaeda leaders, and the methods that interrogators used. The podcast is worth the listen.
People often focus on a sort of oversimplification focused specifically on waterboarding itself. There are other factors which are frequently overlooked. Among these are:
The Geneva Conventions: The Geneva Conventions are often mentioned in the contest of Guantanamo Bay, CIA "black sites", interrogation, and alleged torture. Typically overlooked is the actual text of the Geneva Conventions, in which the treatment of prisoners of war is contingent upon combatants abiding by a set of rules. While no one is advocating for torture, there's a strong case for differentiating between the treatment of legitimate combatants fighting in compliance with the Geneva Conventions, and terrorists and other "enemy combatants" who actively circumvent those rules. (There are also legitimate questions as to whether treating even illegitimate combatants under the terms of the Geneva Conventions undermine the Conventions by eliminating the incentives encouraging restraint in warfare.)
The Clausewitzian Trinity: The legendary strategist, Carl von Clausewitz, enumerated what have come to be known as two different "trinities" that define warfare. His "actual" trinity consisted of Passion, Chance, and Reason. His "secondary" trinity, but the one that gets more attention, is the trinitarian relationship between the government, the military (or, more broadly, the security establishment), and the people. The commentariat often overlooks the fact that in the immediate aftermath of 9/11, the people leg of the trinity supported whatever methods were necessary to prevent another catastrophic terrorist attack. As time passed after 9/11, and with the Bush Administration's credibility damaged in the wake of the invasion of Iraq, the people's broad approval for aggressive measures against terrorists waned. On a broader scale, while Americans are reticent to base their opinions on those of the world at large, actions taken in opposition to international sentiment carry consequences. Globalization throws an asterisk into the "secondary" Clausewitzian Trinity that policy makers flout at their peril.
Perception is Reality: Whether waterboarding and other "enhanced interrogation" techniques constitute torture or not, people's perception that they do becomes the narrative. At some point, the actual details become immaterial, and the value of the potential intelligence ceases to be commensurate with the potential costs. (These costs include educating the public to understand the definition of torture.)
Even if one accepts the arguments that waterboarding and "enhanced interogation" aren't torture, and that they do produce useful intelligence, there are no easy answers, and the playing field changes over time. Defense issues like these require policy makers to balance a variety of competing priorities, and to manage a variety of risks. However, as with most challenging issues, knowledge is power, and informed decisions are better than uninformed ones.