Like I said, to whatever degree you personally believe waterboarding is or is not torture is irrelevant; it's defined as torture by both US law and under international law. If you torture somebody, regardless of if it's a member of ISIS or an innocent Afghani civilian, you will face the exact same legal consequences.
Not only is it cruel and inhumane, it's ineffective - it doesn't work. This is not debatable. So if it's wrong and we know it doesn't' work, why would anyone support it? Psychological studies have been done to answer this question and they all conclude the same thing; people support it to quell the urge for revenge. If you see an organization like ISIS burning people alive and chopping off heads, the natural human response is total revulsion and a want/need to respond. In other words, the argument to support torture breaks down to "They're bad, they do bad things, we need to be just as bad to teach them a lesson". "An eye for an eye" as a Christian might say.
When we start dishing out "justice" based on feelings and emotions, you become the bad guy. When that happens you become a tool to be used for further recruitment purposes by "the enemy". They tell their sons shit like "the crusaders want to steal your lands and rape our women", and the cycle of perpetual warfare continues. In effect, this behavior gets more people killed.