The cooling system of the raw water cooled 2000 series is an unusual design.
Initially, when the engine block is cold, all the cooling water flows from the pump through the perforated pipe in the cylinder head and out to the exhaust elbow where it mixes with the exhaust gases.
When engine heats up, the thermostat opens and allows water to also flow out of this pipe via those perforations into the engine cooling circuit, leaving via the thermostat and the external pipe that runs across the top of the engine to the exhaust elbow.
Seawater enters the front of the cylinder head and feeds into a perforated brass tube in the head.
At the back of the engine this tube is permanently connected to the exhaust manifold by a copper pipe with three right-angle bends.
When the thermostat opens some of the cooling water can now flow through the engine block cooling channels and then out through the thermostat into the horizontal tube above the cylinder head. This is sometimes diverted for hot water generation on board.
If you’re having problems, the first thing to check is the thermostat. It should start to a open at 60°C and be fully open at 75°C. If that is OK, then check the rest of this circuit for obstructions. These pipes (and the elbow itself) can get badly blocked up with soot and salt deposits.