Pretty sure it's welded stainless. I drilled holes and injected foam and epoxy over the years, then sealing with epoxy. The 'dirty water' could be rust, could be old epoxy or foam that mixed with sea water and never cured or some combination. In winter I Re-drilled holes to allow water to drain over the winter and not freeze inside the rudder as the original leaks come from the top around the rudder post.
I even tried bagging the rudder all winter, drilling holes and attaching a shop vac to the holes for hours a day to get the water out. It was an uphill fight at the water returned each year.
CNCPHotoalbum.com has some info about the original rudder construction and these problems. Most surveyors will tell you that there is water in most rudders.
The fix (as mentioned before in this thread) is to take it home over the winter, split it open, clean out all the old foam and muck, inspect the stainless frame (& repair if needed), dry it out, fill the two halves with foam, 3M5200 around the rudder post to seal, glass the two halves back together and you are good for another 40 years.
You can order a new rudder for about $5K.
Or you could just keep sailing as long as there is no delimitation as pretty much every old boat out there has water in the rudder.