Flf, How long has your boat been in the water? Simply drill holes in your rudder! The bottom first. if the water coming out is clearish, then the foam hasn't started to rot. If it's brown like syrup.... Not good. The rudder post in my boat gets a condensation drip into it from the deck plate over it. All rudder posts (imho) should be capped at the top.
If the rudder is still solid and not de-laminated the longer you let it "dry" the better.
Here's another thread that discusses the topic Drilling a rudder