It's a device with electronics.. you could put it the receiver inside an airtight container and water will still get in there..
It's something you'll just have to live with..
If you want to operate on or in water, get an r/c boat!
Seriously, to help prevent failures (as long as you aren't driving in standing water) on wet pavement or mud, spray down everything with WD-40, make sure your receiver box is sealed (like on a t-maxx, put a bead of automotive silicone sealant around the receiver box and then tighten), and make sure your on/off switch is protected (a good shot of WD-40 in there will help).
Good Luck