Here's the truth. Waterboarding U.S. POW's was cited in some war crime trials of Japanese troops after WWII. That's not because waterboarding is torture, but because it is physical and psychological abuse that is prohibited by the Geneva Conventions. These were U.S. soldiers captured in uniform on the battlefield entitled to those protections.
In contrast, KSM is an enemy combatant who deliberately violated his obligations under the Conventions and is not entitled to their protection. Our only obligation was not to torture him and I agree with Liz we didn't.
Had the U.S. Supreme Court not ruled that the United States was bound by the Fourth Geneva Conventions in the case, Hamdan v. Rumsfeld (2006), there would be no reason to go after the Bush Administration. I hope I have it correct, but it is this Geneva Convention, Protocol II (that dealt with combatants in non-international armed conflicts) that needed ratification by Congress to be law. Elements of such treatment were covered by the Uniform Code of Military Justice. Since there was no ratification, the court as usual, bypassed the legislative process and the will of the people.