The first step would be making sure that water doesn't get to the electrical components in the first place. Dielectric grease could be used. Generally there won't be enough water to short out things, and rainwater shouldn't affect the primary (low voltage) wiring at all, but could possibly short the coil and spark plug wiring. A little dab of dielectric grease (mainly on the boots) will help with that. Definitely don't smear either type all over things.
Reminds me of when I was driving my Model A pickup across Kansas back in '68 when we saw a wall of water from a rainstorm coming. When it hit we couldn't see more than about 3 stripes on the interstate and the truck died from the water leaking down onto the coil, which was mounted directly below the hood hinge. I got out, was immediately soaked to the underwear, threw a tarp over the hood, dried off the coil and wiring, and restarted (still with the tarp over the hood) and found the nearest overpass to wait out the storm.
