How wet you get depends on where you are sitting, and there's no way of knowing where the drier seats will be. You may get just a little wet, or you may get bone drenching wet by a cascade of water coming over your head. I usually get the cascade of water when I go on it.
If you want to go on it, and it's too cool, plan it as the last thing you do before leaving the park for the day (if it's that cold, there probably won't be a line). Or, you could wear a poncho, or bring an extra set of clothes.
Karen