I haven't had the opportunity to hunt much standing in water or in this case sitting.
I have a good set of waders but was wondering how cold it would get sitting in water when it's 20 degrees.
20 degrees and no wind? If dressed appropriately (appropriate for me being multiple layers up top and 5 mil neo waders with 1600 gram thinsulate insulated boots, shorts, fleece sweatpants/pajama bottoms, 3 pair of Cabelas wool socks), you'll be much more comfortable in the water than out of it. Particularly if the cold is due to an early season cold front.
If you have some tolerance for cold, you'll be gtg even if your waders leak or you get flooded. If you have a problem dealing with being even slightly cold, use heaters in your boots, gloves, and under your head covering (my preference is a camo balaclava). Or stay home.
Wind is a game changer in cold weather as is wet hands and feet. A dry suit base layer under your waders, neo diving gloves, with head cover and layers up top will keep you comfortable in single digit sub 0 temps.
Keep in mind some folks tolerate cold better than others. If you've had a great hunt in the cold, and stayed out until you lost the use of your hands, and couldnt speak properly cause your face and lips were frozen, you'll know what I'm talking about.
IMHO, the benchmark for a "good" set of cold weather hunting waders would be the Supermag 1600.