Actually, the high-end Marmot bags come with durable water-repellent (DWR) outer shells. They used to use Pertex; I don't know what they use now, but I'm sure it's similar. That is a DWR fabric. (I just checked online; the women's Helium still uses Pertex.)
Take a spray bottle with water and mist a spot on the outer shell of your sleeping bag. (This is a test I always do when buying a new sleeping bag, as soon as I get it home) The water should bead up and not soak in. If it does soak in, treat the entire outer shell with one of the DWR sprays made specifically for down sleeping bags. This is important--don't use a general purpose spray which could mess up the down or not be breathable enough. I use ReviveX Spray On Water Repellent. I suggest following the recommendations for care on the Western Mountaineering website. You'll need to retreat the outer shell each time you wash the bag. Be sure to put the bag in the dryer (lowest possible heat) after the treatment to activate the DWR. Actually, if your bag doesn't pass the spray bottle test, put it in the dryer for about 15 minutes and then try again. Often the dryer heat is enough to boost the DWR.
You definitely do not want a waterproof coating or even a waterproof-breathable (WPB) shell because your body moisture will condense inside the bag, especially when it's near freezing.