Assuming this is not just a big bottom paint bubble, I would say you are looking at either rebuilding or replacing the rudder. Start by drilling a hole in the bottom of the rudder and see how much water comes out.

Repair would involve cutting the rudder apart, fixing any corroded internal structure, and putting it back together. A new rudder can be made by Foss Foam. Here's the link.
http://www.newrudders.com/standard-rudders/