I simply dropped my rudder, and poured the water out by inverting the rudder.
The liquid (I hesitate to call it water) was yucky and black, but there is no obvious delamination, or other issues with my rudder.
The leak appears to have been a crack in the external shell of the spade rudder at the top. I cleaned this area out with a Dremel, and plan to fill it with 5200. I'll let it set for a week before I install. It should be good for another 10 years.