Just in case you aren't clear what water boarding is, here's a video of someone getting waterboarded.
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They can't grasp how it could be that bad. Watching this gave me an idea.
They generally measure resistance to the waterboard in terms of seconds. There is a reason for that. It works in seconds. It's hard to imagine the waterboarding someone for months and thinking they still aren't telling you everything they can think of. Perhaps they don't talk because there is nothing to tell.