Since I, and I assume you, have never been the subject at a water boarding party, your assumption that it does not cause pain is unsubstantiated. Come talk to me after a Jack Bauer gets done with you, until then, I lend little credence to your declaration.
As to the specifics of water boarding you can go to wikipedia.
http://en.wikipedia.org/wiki/Waterboarding
What you’ll see there depends on what you want to see. There are plenty of caveats that cut both ways.