321 SS is good too. And yeah, it doesn't get too hot at the back end if it's coupled to rubber, but may be significantly hotter at the front due to evaporative cooling from the seawater. Diesel exhaust is also generally lower temp than gasoline exhaust as diesels operate unthrottled and run more boost on leaner mixtures over most of the load map than gasoline engines do.
Salt water + exhaust gas is not a fun combo for any metal, though, so throw everything you have at it for corrosion resistance. Can you do titanium?