<< How does one "waterproof" a breaker box ? >>
You could replace it with a fully enclosed type. Very expensive.
<< The dehumidifier uses 6 amps, and that costs money >>
Probably cheaper than the option above.
<< The basement is humid, but there's no water on
the floor. >>
Groundwater is permeating your foundation, so until you deal with that humidity
source, keep the dehumidifier going. Good luck.