If this is a Shimano cartridge BB, then there is no way for water to enter from within the BB shell, only from the outside ends. You can't put more lube into them, they are non-maintainable units.
If the bike has a drainage hole on the underside of the BB shell, then leave it open, even when riding in the rain.

Water and worse, solvent, enters the BB bearings between the black plastic seals and the rotating parts. Keep any solvent away. Never spray WD40 into the seals. Never wipe dirt across the seals. Personally I try to avoid cleaning around the BB. When its new I like to coat any exposed spindle in car wax, then just leave it.