From the Bybee memo:
As we explained in the Section 2340A Memorandum, "pain and suffering" as used in Section 2340 is best understood as a single concept, not distinct concepts of "pain" as distinguished from "suffering"... The waterboard, which inflicts no pain or actual harm whatsoever, does not, in our view inflict "severe pain or suffering". Even if one were to parse the statute more finely to treat "suffering" as a distinct concept, the waterboard could not be said to inflict severe sufering. The waterboard is simply a controlled acute episode, lacking the connotation of a protracted period of time generally given to suffering.
The image above is from the Cambodian museum recording the torture committed by the Khmer Rouge. According to the Bush administration, the Cambodians should not have included the waterboard in a torture museum. It inflicts neither pain nor suffering.