But the absence of a "safe word" is not relevant. Do you deny that water-boarding as part of military training is consented to? I think you're being purposefully disingenuous here. There is no comparison between war-boarding a criminal and water-boarding a military trainee. You yourself have acknowledged that a candidate can always wash out.
I signed up for the USAF for TACP, I had no idea what "waterboarding" was, until I felt that first drop. so yes I "consented" however, I had no idea what I was consenting too actually. They kind of did that on purpose.