I have pea gravel under the swim pans, the rest is a nice compost of duck manure and chopped straw, leaves, etc. That provides an area the ducks can hunt for worms and other things - also makes for nice nap space.
I'm glad you've had good luck with it. I will have to read into this deep litter/compost thing. Luckily, I'm pretty far south, so winters are mild and there's never more than a day or two when it doesn't get above freezing. I'm very new to all this, and it worries me every day. I have a midterm exam I should be studying for right now!