I'd replace the coil and the wires, Once they have arced, the carbon track has already formed. Soon it won't start in dry weather. Then I would make some kind of deflector (or replace whatever is missing from the factory setup) to protect the coil from a direct water assault.

Car manufacturers usually do a pretty good job of protecting the ignition system from a direct water assault, something is missing or deteriorated to the point of not working. You need to repair or replace whatever it is that keeps blowing rain water off the coil. Gene