Nothing is this world is 100% waterproof, and even if it is close to start, water is still the "Universal Solvent".
Heat, humidity, evaporation, condensation, all these quaiities allow water to get into places RATS, and sometimes worms can't even go.
Nothing is maintenance free, except maybe a car battery, and even that has to be "maintained", and changed periodically.
You can't just "set it, and forget it", when it comes to water, and corrosion.