the critical difference being that american servicemen are waterboarded in training scenarios where they are well aware that they will experience the sensation that they are drowning but will not be under any real danger.
detainees that are waterboarded have no such pre-procedural assurances and have no confidence that we would NOT drown them... and, in fact, if we kept the water pouring for any extended period of time, they would be in real danger.
and in some future conflict with some other nation state, when our GI's are captured on the battlefield and interrogated...if they are are waterboarded they will not know whether or not they will survive it.
My PRIMARY objection to any sort of extreme interrogation technique - besides the fact that it may very well be unconstitutional - is that however we treat our detainees in THIS war will send the message to any future enemies that we have absolutely no problem if they use those exact same techniques on our boys. They may, in fact, get treated much worse, but we have absolutely no right to expect that they will be treated even one iota better.