My only experience of overheating in water cooled, diesel marine four bangers has been ...
BMC 1.5 - Raw water feed, rubber pipe blocked between inlet strainer and pump.
Externally, the integrity of the hose looked intact. The pump was suspected as no blockage was evident in the feed pipe.
It transpired the pump was fine, it was the inlet hose. Although the outer winding looked strong and intact the inner core had delaminated and was squashing flat under the negative pressure from the pump. However with the engine stopped the pipe would reform its shape.
Dorothy Perkins 4109 40hp - Thermostat misfunction leading to an unexpected failure in the closed position.
When starting from cold, temp gauge would shortly become erratic in operation. Likewise wet exhaust fluctuated between water and steam, eventually strangling engine of all cooling.
It was concluded that the thermostat was perhaps a post market product of poor quality or not the correct one. Resulting in failure in a closed position instead of open.