They sell belts for waders that you put around the wader at chest level to stop the water from getting in. I have one but have never been in a situation to see how well it would work as I just use it for surf fishing.
http://www.cabelas.com/product/Cabel...h-All+Products
If you 'accidentally' tip over and your wader fills with water you may not sink but you also won't be able get back on/in your kayak. Saltwater weighs 8.5 lbs per gallon and I have no idea how many gallons a wader can hold. :-)
Normally I just use a farmer john. Easy to pull down if I start overheating.