It is not against the law under certain specific circumstances. Like, if you are a CIA professional, and the person that you are getting info from is a terrorist, and the president has given you permission (and congress has been briefed and given the go ahead on the procedures).
That is a legal opinion, not a law. Those who employ it are probably in violation of federal, military, and if applicable, state law.
Other that the former president, vice president, and the CIA? If President Obama was so convinced that it didn't work, why did he retain the right to authorize the technique in the future?
Two people who never saw or used it in person and those who advocated for its use in the first place, of course they are going to say it works. Those who have employed it for decades in training, JPRA, say it doesn't. I'll go with the experts.
OK, so torture does not work. I'll give you that. I also think that the folks employed at the CIA know a thing or two more than you do about getting quality information out of people. They developed the exact techniques, and put them into action. The CIA is not in the business of doing things that they know will not work. They got the info that they needed so the FACTS are that waterboarding did work. If you want to cling to the idea that "torture does not work" fine. What we did to those guys did work and lives were saved. Must not have been torture. How convenient.
I am not a trained interrogator or debriefer but I have known several people who are/have been and all have said it does not work.
Before 9/11 the CIA was not in the business of capturing, jailing and interrogating people. They were more in the business of 'debriefing' cooperative persons, like defectors or cooperative sources. The people who were responsible for interrogating terrorists was the FBI and other federal law enforcement. Instead of relying on tried and true methods of interrogation the CIA asked for information about methods our former enemies used, with very mixed results. In reality, the CIA was doing many things on the fly after 9/11. I wold presume things are better now and surprisingly, or not, they have stopped waterboarding people. Figure that.