Beyond that, I'm not sure how the exhanger's bolt functions with what's left. Is it steal sealed between the coolant and the seawater sections?

Is the treated rod continuously threaded -- or is like a carriage bolt (smooth) with threading only on the end. If you're clear of the threading, then you might try to punch it out AFTER using some penetrating lubricant. IF it's not clear of the threading, that would obviously NOT be the way to go.

Is the section that you broke-out just corroded on the surface -- or is it the body of the exchanger that is coroded around the the "okay" rod?