My prior L25 had the same problem. It may be too late, given current weather, but I would certainly advise you drill at least a few holes, through the rusty areas, to allow the water a place to go. Otherwise it is quite likely that your rudder will literally start to split in half. Mine did the one year I failed to make drain holes.
I had a heck of a time trying to figure out where the water was coming from, one obvious and easy place to address is to seal around the juncture of the rudder post and the fiberglass at the top of the rudder with a high quality silicone sealant. I could literally watch water run down the post to the top of the rudder and that junction is a ready access point to the rudders interior.
I also suspected that water was condensing on both the outside and inside of the hollow rudder post. Perhaps others can provide more in depth experience with this, but had I kept that boat, I would have considered filling the rudder post with epoxy or closed cell foam to slow that. The thought behind this is that, in cold waters, the rudder is submerged in 60-ish degree water causing condensation.
You may wish to contact Foss, in any case, as it is possible that they made your rudder for Cal, or that they are familiar with its composition. It is critical that you understand how the torque is transferred. Ideally is should be something similar to a stainless plate that is welded to the post along a long length. Non-ideally it is a few lengths of stainless tube or post welded to the rudder post like fingers. Those fingers can rust and snap off leaving your post spinning in your rudder. It is certainly doable to repair your rudder rather than go to Foss, but I wouldn't give any credence to recommendations of a drill and fill approach (drilling your rudder like swiss cheese, drying it and filling the holes). Pull the rudder, cut a large window in one side and dissect it. Plan on replacing all the foam with new closed cell foam and having any necessary welding done (I liked the flat plate that Lancer used with a hole in the center for weight reduction...that way the entire length of the plate is welded to the post...I'd ask Foss if they could do that if they were making one for me), then restoring the skin. Once you consider all that work, plus the fact that the rudder post may be questionable, Foss Foam's price will probably look pretty reasonable (about $2-3k).
