Whats the scoop? Engine running hot? Overheating? Did you change the salt waterpump impeller? Fresh water pump working? Is water coming out exhaust when running? have you opened and cleaned the intake strainer? Made sure you have good water flow through strainer?
Perkins have a bullet proof reputation. The word is, keep the oil changed and full, the water in the block and circulating and the filters changed and they last for 10000 hours.
Have you shot the water channels with a temp gun to see delta on intake, and output of 1. main heat exchanger, 2. transmission, and (surprisingly) the hot water tank. It all flows through all three. If one is impinged it slows down all heat transfer. For my money (after you ascertain waterpump is circulating) disconnect hot water heater from equation. Then check coolant flow around engine and gear box exchanger. Eliminate the outside issues first.
Although I haven't seen the problem personally, hearing about it (anecdotally) about the rubber impellers disintegrating and clogging the heat exhanger tubes (and thus down flow restriction) is an issue. Disassembling the waterpump and seeing the full (or lack of) impeller makes this a (non) issue as appropriate.
Start with the simple stuff first. Waterflow. Impeller. Circulation. Clogs.