http://people.howstuffworks.com/water-boarding.htm
CIA members who've undergone the technique as part of their training have lasted an average of 14 seconds before begging to be released.
The Navy SEALs once used water boarding in their counter-interrogation training, but they stopped because the trainees could not survive it without breaking, which was bad for morale.
When the CIA used the water-boarding technique on al-Qaeda operative and supposed "9/11 mastermind" Khalid Sheik Mohammed, he reportedly lasted more than two minutes before confessing to everything of which he was accused. Anonymous CIA sources report that Mohammed's interrogators were impressed.
Most CIA officials say water boarding is not torture, although many see it as a poor interrogation method because it scares the prisoner so much you can't trust anything he tells you.
Senator John McCain, who was tortured as a POW during the Vietnam War, says water boarding is definitely a form of torture. Human rights groups agree unanimously that "simulated drowning," causing the prisoner to believe he is about to die, is undoubtedly a form of psychological torture.
The international community recognizes "mock executions" as a form of torture, and many place water boarding in that category. In 1947, a Japanese soldier who used water boarding against a U.S. citizen during World War II was sentenced to 15 years in U.S. prison for committing a war crime.
What is it?
Water boarding as it is currently described involves strapping a person to an inclined board, with his feet raised and his head lowered. The interrogators bind the person's arms and legs so he can't move at all, and they cover his face. In some descriptions, the person is gagged, and some sort of cloth covers his nose and mouth; in others, his face is wrapped in cellophane. The interrogator then repeatedly pours water onto the person's face.
Depending on the exact setup, the water may or may not actually get into the person's mouth and nose; but the physical experience of being underneath a wave of water seems to be secondary to the psychological experience. The person's mind believes he is drowning, and his gag reflex kicks in as if he were choking on all that water falling on his face.
It's a form of torture, hands down.
If even professionals who are trained to handle torture, cannot stand it, it's got to be a horrible experience.
(I missed where sixofnine describes it, sorry)