I have that kind of exhaust too. Mine is Stainless—just painted copper. I've read that when they fail you have no warning and the water just comes in through a hole or crack from the jacket to the exhaust.
Mine exits the boat high above the waterline and no siphon is involved from the exit end. If you close the seacock then no seawater can come in.
I think you have it figured already
Russ