??I don't think there is a PFD made that would keep me afloat if I tipped the yak and flooded my stocking foots.?

Dave,
What would make the water inside your waders different then the water outside? Once filled, it wouldn?t matter which side of the waders the water was on, you would float (or sink) exactly the same. The weight of the water in your waders would only be a factor if you were not in the water. There are several good reasons not to Yak in coldwater, hypothermia for instances, but filling your stocking foots would not affect the performance of your PDF.