exhaust elbow

While I was on the boat at the weekend, I noticed a little pile of salt under the engine..... there'd previously been a leak from the impellor face plate, which the previous owner claimed to have fixed.... so I was groaning.... however, upon exploration, I had to take back my curses, as the leak was in fact from the exhaust elbow, or more accurately from the water injector pipe to the exhaust elbow, which is a small and seperate casting, and appears to be pretty badly corroded....So, i'll replace this section.... lots and lots of dried (ish) salt around the area..... maybe a replacement elbow will let a bit more water through the exhaust and stop the slight case of steam that we have....I just need to get some measurements so that I can order the right bits....