Torture is wrong;
water boarding is torture, therefore
water boarding is wrong
water boarding is torture, therefore
water boarding is wrong
It is never permissible to do what is wrong to obtain a desired good end. In other words, ends do not justify means. On my understanding, ethics are to morals what technology is to science. Stated more clearly, ethics is about applying morals. If you get the morality wrong, it is most likely that you will do something wrong. Morality needs to be grounded and pragmatism is not a sufficient ground for morals, which is why, even if embryonic stem cells led to therapies and cures (this is far from certain), it is still wrong to create life in order to destroy it. Now, I will accept that ethics, applying morals to life, requires a certain degree of pragmatism, but never to the degree that results in it being okay to do what morality forbids.
So, the take away here is that even if this particular "enhanced interrogation tactic" yielded actionable intelligence, it was still wrong, a contravention of U.S. and international laws. Think of the precedent this sets across the board, at home and abroad.
We must never forget, as Giussani reminds us, that "[t]he source of being moral is loving someone, not fulfilling laws" (Is It Possible to Live This Way?: An Unusual Approach to Christian Existence, vol. 2, Hope, pg. 117).
Veni Sancti Spiritus, veni per Mariam.