It would seem that American-style waterboarding of prisoners for information is not torture. Torture causes persistent physical and/or psychological damage to the victim. But American-style waterboarding of prisoners for information does not cause persistent physical and/or psychological damage to the victim. Therefore, American-style waterboarding of prisoners for information is not torture.
I reply, torture is physical or moral violence directed against the bodily integrity of the victim, as the Catechism of the Catholic Church says (n. 2297). But offenses against bodily integrity need not cause persistent effects, as is shown by the fact that kidnapping and hostage-taking are also offenses against the bodily integrity of the victim, and they need cause persistent effects no more than waterboarding. So the absence of persistent effects is not proof that an act is not torture.
A speculation: People often think of torture as necessarily causing persistent injury because most methods of torture actually do, or at least can, cause persistent injury. The combination of a will to torture and a will to avoid persistent injury is relatively rare in history.
An observation: I didn't mention this in my reply because it's not necessary to refute the argument in the first paragraph, but: The truth of that argument's other premise -- "American-style waterboarding of prisoners for information does not cause persistent physical and/or psychological damage to the victim" -- is by no means established. In particular, it is not established by anecdotal accounts of American-style waterboarding of SERE trainees, for two reasons. First, among the many circumstantial differences between waterboarding prisoners and waterboarding trainees is the difference in relationship between the waterboarder and the waterboardee, and among the differences that follow from the difference in relationship is that being waterboarded is a different psychological experience for a prisoner than it is for a trainee. The second reason anecdotal accounts of SERE trainees does not establish that prisoners do not suffer persistent injury is that some anecdotal accounts of SERE trainees include persistent psychological injury.