I don't think it is.
Here's my litmus test for torture: if people of sound mind willingly volunteer to undergo something, it's not torture. And people of sound mind regularly volunteer to undergo waterboarding -- mostly for SERE training, but in other instances as well.
Are you familiar with anybody who has volunteered to try out an acid bath? Me neither.
Proven? You do realize that
Leon Panetta publicly let the cat out of the bag about KSM's waterboarding being the source of at least some of the intel that led to locating OBL. And what on earth does "not...all that effective" mean, anyway?
Look, if it was actually ineffective, then the entire conversation would be moot...because nobody in the intelligence community would even want to employ it. You think they want to do things which don't work....or, worse yet, produce faulty intelligence?
I don't have a strong opinion on its effectiveness. I'm in no position to have one. There certainly are intelligence pros who say it isn't -- there are also intelligence pros who say it is. I'm content to leave that to them to decide -- depending on the particulars at any given point in time.
What happened to the prosecutions, then?
Tell Osama that.
As I said in that post, I think reasonable people can disagree about this. But few people who do disagree with me on it seem to share that sentiment.