To clarify about your Gitmo point: the vast majority were not waterboarded. Going to Gitmo did/does not automatically mean being waterboarded. But point taken.

I thought of another good argument against waterboarding: the Geneva Convention. My understanding is that it is banned by it under all (or, well, proscribed) circumstances, and the US is signatory to it. I think part of the reason why the US wanted to do certain things NOT on mainland US soil was to be able to weasel around certain laws and international rules that address what they are and are not allowed to do.