As Fernarias mentioned earlier in the thread, some components can allow water to infiltrate, and if the water has any impurities in it, the water plus impurities may cause corrosion and deterioration. Personally, I'd be worried about water in the transformer.
The longer the unit sits, especially if water has entered some components, the more likely there is to be some corrosion. If you've done everything you can to dry it (moving air, gentle heat, or using some sort of dessicant, not sure how to spell that, I'd go ahead and turn it on.