I've posted this before on previous harness threads and it mostly gets ignored, but I still believe in it whole-heartedly:
kayak rescue vest
By clipping on off your back you WILL NOT go under or face down while dragging through the water. Clipping on off your chest just seems like asking for trouble. Ever try water skiing and not gotten up or let go of the rope? That's what clipping in off your chest will be like over board. Of course, having your tether short enough to not go into the water is a good idea, but you never know what will happen.