It's a good question with, I think, two separate parts.
- is waterboarding a form of torture?
- can the use of torture ever be justified?
On the first point, it's very difficult to argue that it's not a form of torture in that it causes severe physical and mental pain and has been found to be torture by leading legal experts, politicians, military judges and intelligence officials.
On the second point, the international community (including United Nations, European Union and the Geneva Convention) has been absolutely unanimous in that freedom from torture is a fundamental human right. Not only that, it is one of the very few (alongside the right to life) from which there can be no derogation even in times of war.
I'd say that it, however, possible to justify the use of torture (conceptually) on very specific grounds, in the same way that you abrogate from the right to life, e.g. if an armed policeman sees someone who is about to shoot somebody else, they have the right to shoot and kill that person (an extension of the self defence argument). In principle, I imagine that you could make the same argument for torture, i.e. that if the use of torture will prevent an imminent and specific crime that would result in the death of someone else then that would be potentially justifiable. On a practical basis, however, I suspect that there would be virtually no cases that would meet those criteria.