I normally wear a waterproof top of one type or another, depending on temperature as I don't have a long raincoat. It is also important to keep your kilt from drooping. I suspect one of the reasons military kilts tend to be shorter is so that if they get wet, they are not slapping against the back of your calf when you are walking. One that only touches the leg a little above the back of the knee is much more tolerable when wet.