I don't think Traxxas made the E-maxx waterproof so people could submerge it. Well, atleast not just drop it into a swimming pool or something.
From the looks of the E-maxx page, the entire truck is waterproof. It says the recievers waterproof.
Sealed Watertight Receiver Box
A ground-breaking new weather-sealed receiver box is another Traxxas innovation that puts E-Maxx in a league of its own. The antenna and servo wires route neatly a special watertight seal (patent-pending). To protect the receiver from dust and water a custom shaped blue O-ring seals the receiver box. Thanks to the unique design, the receiver can be removed for service and even aftermarket receivers can be used, without losing the sealing properties of the box.