More to the point than the Geneva Conventions: the UN Convention Against Torture, which is exactly what it sounds like. As for whether waterboarding is torture

More to the point than the Geneva Conventions: the UN Convention Against Torture, which is exactly what it sounds like. 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.)