Here's a great summary of what waterboarding does:
Terror is a simple business. All you have to do is drip water into a cloth held over a man's face, and he will feel - he will know - that he is drowning. It is false knowledge, yes, but it has the force of truth. And to know that you are dying, being executed, is an astonishingly terrible thing. It changes a man, much as rape changes a woman.