Death Valley is a great spot in the winter and hold lots of good cycling. The snow level is around 6-7k and the nights get cold (30's overnight) a bit warmed in the valley. Plan on traveling on both dirt and paved. All the dirt roads are just that, roads. Don't plan on any singletrack. Any passes over 6k will have snow but everything below will be in excellent riding condition. The best place for info is google earth and the death valley park website. There are some other sites out there...i think deathvalley.com is one. Google earth is the best...the use Topofusion to fine tune. Like you mentioned...water is the major issue. The only reliable water is at established facilities, springs and creeks are almost non existent
Another spot in Cali. for super winter riding is the Mojave nation Preserve. This guy does a lot of riding there and it might be worth while checking out (
http://mojave2011.drycyclist.com/main.php). The Mojave will have much more options for sustained dirt adventures with water resupplies within a reasonable time. And the scenery is bitchen because it is the contact zone of three great deserts...the Sonoran, the Colorado, and the Mojave. Its an amazing unknown place.
Where will you be traveling from and when will you be riding? I play in DV every winter so will be glad to help if needed. Either place DV or Mojave will surely be an excellent adventure. Enjoy.