My Bristol has a seacock on top of the CB trunk aft. You take the plug out of the seacock and screw in a threaded aluminum tube with a large piece of dowel in it. Then you open the seacock and can bang on the end of the dowel to free the CB. Never actually had to do this, but it seemed like an elegant solution provided by the builder. No problem doing it with the boat in the water if need be. Perhaps you have something similar or should install one.
In South Africa doing lots of boat stuff. Departure north from Cape Town around December 15th. Arrival in the Caribbean around the beginning of February, after 5300 miles or so.