Funny, but I was just looking at a post from two years ago, where I rebuilt my drive line, and I had to drop my rudder. You can read about that here: Contemplating a drive line overhaul
What I did not delve into in that post was how I addressed the water intrusion.
It was clear to me that the water was penetrating the rudder, where the rudder shaft met the rudder, here;
The red stuff is not corrosion, but red grease for the rudder tube.
You can't see them, but there were small stress cracks around the shaft (or is it the stock?
... whatever...) I believe that water was penetrating these small cracks, and travelling down next to the tube, and collecting in the bottom of the rudder.
What I did to address this is I took my trusty Dremel and routed out a ½" deep, ¼" wide channel in the rudder around the stainless steel tube. The channel was "V" shaped. I then filled this trough with the dreaded 3M 5200, and set the UHWMP spacer that you see in the pic above in the 5200. Think of it as bedding the spacer. I then re installed the rudder on the boat.
In the past two seasons, all has held up fine.