The person writing in the link below wondered this so he administered it to himself. His conclusion? It's torture. In fact he has this to say

"So, is it torture?

"I'll put it this way. If I had the choice of being waterboarded by a third party or having my fingers smashed one at a time by a sledgehammer, I'd take the fingers, no question.

"It's horrible, terrible, inhuman torture. I can hardly imagine worse. I'd prefer permanent damage and disability to experiencing it again. I'd give up anything, say anything, do anything."

Is Waterboarding Torture?