I saw the waterboarding device in Cambodia's notorious Tuol Sleng prison in Phnom Penh and did not see another until I was strapped down on an identical one at SERE. Waterboarding was administered as a 'stress demonstrator' to show that an enemy could make one say anything. And one does.
The attacks of September 11 were made possible, in part, by the fact that U.S. law enforcement had a bias that 'rich Saudis are safe Arabs.' Al Qaeda understood that bias and relied on it to operate freely in the United States for years, even when the terrorists were engaged in activities that the authorities considered suspicious.
To help students steel themselves for captivity, SERE used a variety of 'stress and duress' techniques. The military's encyclopedic knowledge of these techniques was paid for in American blood because it was gleaned from former POWs tortured by totalitarian regimes. One technique, waterboarding, was a historically well-known torture.