If you are talking about a marine engine, first check the salt water circuit, check the salt water pump, generally driven by pulleys or distribution.
These pumps, if girls, are with rubber rode and fail at the same time.
You can also have the water suction full of bugs in the keel.
Remember that marine engines do not have a radiator, but heat exchanger, so it may be that your engine does not manage to get heat from their refrigerant.
Check the status of sacrifice anodes and replace them if necessary, otherwise your engine is corrode