 Well once Dunn is knock out it isn't all that hard to try several different ways to kill him. Further it doesn't even have to be water. A pillow over his head after you hit him with a claymore mine should do the trick. No, it is quite clearly stated that it is only water that can suffocate him, nothing else, as he swallows it too fast and panics whilst submerged in it. Simply stuffing his mouth with a gag or something along those lines wouldn't kill him, only water. 