I wouldn't worry about a bit of water at all, the system uses two trap doors actually, after the first(outer) one there is a compartment with a drain hole at the bottom, then another trap door that actually goes to the tank. That is why GM includes the little funnel with the trucks, to get past the second trap door, and trust me, it's needed for adding fuel de-icer or additives to the tank, and not the ground below the truck.