I understand that we are talking about river wading. However, I think an issue that comes up for me when fishing the salt is somewhat pertinent here.
In an attempt to reach breaking fish I quite typically inch forward into deeper and deeper water; most of the time, I'm not even aware I'm doing this. Eventually I arrive at a point of diminishing returns -- that is, a point where I lose casting form and leverage on the rod, and distance starts to suffer. I find I can actually get the fly further out by backing up a few feet thereby increasing the percentage of my body that is above the water's surface. The margin for this point is pretty narrow. One or two inches is all it takes.
For me, the point of casting melt down is right at my belly button. I'm 5' 9" with very short legs (29" inseem). I imagine taller fishers have a significant advantage here, especially if they have long legs. (Perhaps this explains the extraordinary fish catching abilities of the Estey brothers
My point here is that sometimes wading deep is actually the wrong thing to do regardless of any safety issues. This doesn't take into account such problems as room to back cast, but it is something to keep in mind on big water with open space.