A conscientious DOJ attorney who is contemplating the possibility of a successful prosecution under the federal torture statute will undoubtedly be aware of the talking point that the Americans executed Japanese war criminals for waterboarding. Mark Hemingway has written on this topic at National Review Online. For the moment, let's put aside any factual differences in how the waterboarding was done by the Japanese and the Americans. If you follow Hemingway's links for the war criminals who were executed, and then click the "legal procedure" tab, you can find the particular crimes for which they were convicted. Some of them were convicted of crimes related to mistreatment of prisoners of war. But these convictions are for violating the laws of war. Under the laws of war, prisoners of war are not supposed to be subject to degrading treatment (e.g., wall-standing, noise) nor to torture. Accordingly, the convictions for the seven executed Japanese do not imply a legal determination that waterboarding by the Japanese constituted torture, as opposed to degrading and inhuman treatment.
If you want to make a case that it is a crime under federal law not to treat al Qaeda terrorists as lawful prisoners of war (which means that they are not even supposed to be interrogated against their will), go ahead and try. But that's going to involve a different statute (if you can find one) from the federal torture statute.