Sorry but using cruise control in heavy rain is just simply a bad idea. It's supposed to be used when conditions are ideal, not when they as bad as can be. That is human error, in my opinion.
I'm struggling to think of any combination of hardware/software that isn't asinine on first glance (e.g. feed the cruise control off only the rear wheel speed sensors on a FWD vehicle) where cruise control would be any different than a driver manually holding the same speed by putting their foot on the gas.