I tend to view the "don't wear waders in a yak" thing as the fishing derivative of an urban legend. Waders (neoprene, anyway) are *definitely* the way to go in cold weather and/or water. First of all, closed cell foam (neoprene) floats, so you'll actually get a little extra flotation out of them. Make you clumsy? Sure, but whatever water goes in will NOT weigh you down while you're in the water. What they will do is to provide some insulation, which buys you time to get back into the yak. Wading belt? Yes, but primarily to minimize the intake of cold water.
This time of year, I wear my neoprene waders, a fleece, and a wind shell over the top. (PFD too, of course). Works like a charm.