The Syosset superchargers are built next to a bunch of plantings that are on timed sprinklers.. the car enjoyed the light wash, and was happily topping up at 280 mph.. I on the other hand, got drenched.
I don't think rain is an issue for supercharging (or any sort of charging) otherwise these chargers would not be anywhere near sprinklers.
I can't say I was't apprehensive about it, but I needed the charge to get home.