The CIA has declassified documents related to its Bush-era waterboarding program, and the efficacy was not impressive [theguardian.com]:
Medical personnel who aided in the CIA's first simulated drowning found that the detainee who endured it was probably willing to cooperate before his torture. They even mocked agency defenses of waterboarding as "creative but circular", according to an extraordinary declassified document.
In an assessment devastating years of CIA and allied insistences that waterboarding was a critically important intelligence-gathering technique, the undated and heavily redacted document attributed to the chief of the CIA's Office of Medical Services (OMS), states that detainee Abu Zubaydah's cooperation with interrogators in 2002 "did not correlate that well with his waterboard sessions [cia.gov]".
The Abu Zubaydah waterboarding was the wellspring from which the CIA's infamous former torture program, which it refers to euphemistically as "enhanced interrogation", emerged. Contractor psychologists Bruce Jessen and James Mitchell, now the subject of a federal lawsuit [theguardian.com], used it as a test case to implement a program they designed that yielded international human-rights condemnations and an extensive Senate committee inquiry. Eighty-three times in the span of a month, CIA interrogators subjected Abu Zubaydah to waterboarding, a technique the body processes as drowning-induced suffocation. But writing in "retrospect", OMS concluded that Abu Zubaydah "probably reached the point of cooperation even prior to the August [2002] institution of 'enhanced' measures – a development missed because of the narrow focus of questioning".
One conflict of interest: psychologists who had a say in whether interrogation should continue were paid $1,800 per day [cbsnews.com].