In an earlier post I noted that the United States has prosecuted waterboarding -- the practice of strapping prisoners to boards and pouring water into their faces, historically also called "water torture" and more euphemistically "water treatment" or "water cure" -- as a war crime. I've been asked to provide a little more information on that, so here's a brief summary and some links. You can easily find many more with your favorite search engine.
Prosecutions date back at least to one against an American officer in the Philippines in 1902, though that apparently involved an even more brutal form of the practice. A contemporary New York Times article can be read here.
At a St. Petersburg FL campaign event on November 29, 2007, Senator and former POW John McCain noted that " ... following World War II war crime trials were convened. The Japanese were tried and convicted and hung for war crimes committed against American POWs. Among those charges for which they were convicted was waterboarding." The Tampa Bay Times looked into this in some detail and verified Senator McCain's statement. To be clear, not all of those convicted were sentenced to death. For example, Yukio Asano was sentenced to 15 years hard labor for waterboarding and other crimes. According to the Encyclopedia Britannica, "the International Military Tribunal for the Far East (IMTFE; 1946–48) convicted 25 Japanese leaders of responsibility for war crimes and crimes against humanity, specifically including torture by waterboarding (referred to by the IMTFE as the 'water treatment')."
Some have pointed out that the lists of crimes in these cases included things in addition to waterboarding. That's true, but rather beside the point. If someone is convicted of a set of crimes including rape and murder, that doesn't mean rape itself or murder itself isn't a crime. There are also quibbles over the details of the technique, such as whether a cloth is placed over the prisoner's mouth and nose. Either way the prisoner is kept from breathing, and people who have actually undergone waterboarding pretty consistently agree that it's torture.
One more example: According to an ABC News article on waterboarding, during the war in Vietnam, a photograph of an American solidier participating in waterboarding a prisoner was published in The Washington Post and that soldier was in short order courtmartialed. I wasn't able to find more details on this instance, but I confess I didn't have time to research it in any depth.
If the U.S. prosecuted a member of the Taliban for waterboarding a captured American, I'd be surprised -- and certainly disgusted -- if any defenders of waterboarding objected.