This is my Traxxas Slash running a sealed XL-5 and a Stinger 20T motor. Aside from an Airtronics radio, everything else is stock. We had a massive ice storm recently, and there was some charge still left in the battery from the day before. The answer to the question? As waterproof as your battery.
I have added the part numbers for my tires and rims since a lot of people have asked about them. The note is found at the 1:00 mark.