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.
As for whether waterboarding is torture under US law: the US executed a few Kempaitai (Japanese secret police) agents for torturing US prisoners in WWII; they had waterboarded them. Waterboarding is also covered under UNCAT.
(Note to anyone assuming a partisan motivation: I am a serious Catholic, thus not by any stretch of the imagination a US liberal.)
There are several "Geneva Conventions". The US has signed some of them.
In general, the Geneva conventions only apply when both parties are signatories. Since the Taliban and such orgs aren't signatories ....
And the conventions quite definitely do apply to some degree when a signatory fights an opponent that is not a nation (and which therefore, by definition, cannot be a signatory). The details are a bit difficult, especially when a conflict is both internations and involves non-nation parties, but it's definitely not correct to say "our opponents are not signatories, so all rules are off".
It happens even when the US government (or state or local law enforcement authorities) kidnaps and enslaves native US citizens in the course of enforcing domestic criminal law. Later, some of these people get released. Some are found innocent. Some are not but the cases were flawed or some legal rule was not followed to the letter. And sometimes these people are found guilty by a court, serve sentences (in a prison as slaves) and are then let go, sometimes many years later. This is normal in the US and throughout the world. Unfortunately.
Heck, yes. Or pretty close to it. Quite a few Gitmo inmates were completely innocent, they just were at the wrong place at the wrong time, or had the wrong surname, or had enemies who called them terrorists to get rid of a business rival, or to get the bounty that US was paying.