When you ride, water is evacuated quickly by the pumps. When it rains hard and you are not riding, your ski stays still (of course). This is why I always put the cover on it when I am not around to check it after thunderstorms or a few days away from my ski.
Some people will tell you never to leave your ski in water when not in use. A bit tedious but it is the ultimate remedy. I guess you could test it: pour some water over it and observe for a few hours.
Benji.