Here is Chuck Schumer's justification for supporting Mukasey:

This afternoon, I met with Judge Michael Mukasey one more time. I requested the meeting to address, in person, some of my concerns. The judge made clear to me that were Congress to pass a law banning certain interrogation techniques, we would clearly be acting within our constitutional authority. And he flatly told me that the president would have absolutely no legal authority to ignore such a law, not even under some theory of inherent authority under Article II of the Constitution. He also pledged to enforce such a law and repeated his willingness to leave office rather than participate in a violation of law.

Congress did pass a law banning "certain interrogation techniques" - it banned all forms of torture.

What is torture? Under international law it is

"any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person."

Congress explicitly prohibited torture twice.

First, the Senate ratified the Third Geneva Convention (GCIII) on July 6, 1955, which protects prisoners of war against torture. And those who say Al Qaeda is not covered by the Geneva Conventions are wrong. GCIII includes Common Article 3, which protects all prisoners, including "enemy combatants." According to the Center for Defense Information,

The U.S. Supreme Court’s June 29, 2006, decision in Hamdan v. Rumsfeld applied Common Article 3 to a global conflict with a non-state actor, al-Qaeda, taking place within the territory of a country that is a party to the Geneva Conventions, Afghanistan.  Its implications are that Common Article 3 applies to the global conflict with terrorists anywhere on earth involving the territory of a party to the Geneva Conventions.

Second, in 1996 Congress (then led by Republicans Newt Gingrich and Trent Lott) enacted the War Crimes Act of 1996, which defines a war crime to include a "grave breach of the Geneva Conventions," specifically including torture.

In the wake of the Supreme Court's Hamdan ruling, Congress adopted the Military Commissions Act of 2006 to remove habeas corpus protection from "alien unlawful enemy combatants." It also amended the War Crimes Act of 1996

so that only actions specificly defined as "grave breaches" of Common Article 3 could be the basis for a prosecution, and it made that definition retroactive to November 26, 1997. The specific actions defined in section 6 of the Military Commissions Act include torture, cruel or inhumane treatment, murder, mutilation or maiming, intentionally causing serious bodily harm, rape, sexual assault or abuse, and the taking of hostages.

So despite MCA, torture is still a crime, and the definition of torture has never been changed so it still covers "any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person."

And does this include waterboarding? Here's a description by Malcolm Nance, who trains American soldiers how to resist waterboarding.

“Waterboarding is slow-motion suffocation with enough time to contemplate the inevitability of blackout and expiration. When done right, it is controlled death. When performed with even moderate intensity over an extended time on an unsuspecting prisoner – it is torture, without doubt. Most people cannot stand to watch a high-intensity, kinetic interrogation. One has to overcome basic human decency to endure watching or causing the effects. The brutality would force you into a personal moral dilemma between humanity and hatred. It would leave you to question the meaning of what it is to be an American."

If Chuck Schumer doesn't think waterboarding is covered under current law, he needs to resign from the Senate immediately.