Fox News had a segment on waterboarding recently where the reporter got subjected to waterboarding, but it was pretty tame. I came across this video from CurrentTV which portrayed it quite a bit more graphically. There's also a longer, "uncut" version on the upper right.

If you're opposed to waterboarding, this video won't change your mind. It's pretty brutal. But I don't think you can have a proper debate about it without really understanding what it entails.