No.
A “reasonable person” would be unable to avoid the knowledge that his/her actions waterboarding someone was, in fact, severe to meet the definition of the law.
As mentioned maybe the first time you do it you could make that claim as a defense citing you simply did not know or have any reasonable way to know what you were doing caused severe anguish. Indeed others told you it was ok so you did it.
Fine. You do it. NOW you are faced with the inescapable conclusion that you are, in fact, causing severe mental anguish. Any “reasonable person” would HAVE to come to that conclusion. It is unavoidable unless you are mentally deficient. At this point, if you continue, you are now in violation of the law. No appealing to some attorney telling you it was ok should save you.
You have no basis for this assertion. Again you display your notion that a more overtly violent torture such as clubbing is “worse” despite being shown people who are waterboarded have been known to struggle so hard they break their own bones fighting against the restraints. That is the very definition of agony to me.
Maybe clubbing is worse, maybe not. Without testimony from a few people who had both done we cannot say. We can say however that waterboarding is severe and in the same ballpark at the least.