The engines should have an elbow
at the end of the exhaust manifold where the sea water exits the exhaust manifold and is injected into the exhaust stream. It's common in older engines for rust and other debris to partially plug
causing reduced flow. But several things can cause flow variances between to engines. Strainers could be plugged, heat exchanger
could be plugged, bad impeller, intake valve partially closed, collapsed hose, muffler
blockage and so on.
Some engines that have larger sea water pumps have a diverter that sends some water over the side w/o going thru the exhaust system. Usually controlled by a valve that is partially closed. Open - most of the water bypasses exhaust, closed - all the water goes out the exhaust.