blu-tac the mode and set buttons, and the LED holes.. anywhere water might be able to creep in.
if you can, mount the speedo on 3 or 4 bits of servo tape so if the worst happens it will not be 'sitting' in water on the chassis.
the biggest thing is making the best seal between your body and your chassis, once your car is good to go, duck tape the body on.
raise your ride height too, it does help
try and race a 'dry' line
but, the main thing is, YOU have to choose if you want to risk your equipment or not.