Here's something to remember about waders. Even if they are FULL of water, they still do not weigh any more than they did on dry land when you pulled them out of the box... The water that fills the waders does not weigh anything, (compared to the water surrounding the waders) until you try to get out of the water...
The best thing you can do is to tuck your knees up to your chest, this will keep air in the leg portion of the waders which not only provides boyancy, it also reduces your exposure to the fridged water in that portion of your body. If you have a life jacket on, you will be able to hold this position for quite a while.
Try it for yourself this summer, and hope you never have to use this information.