There are two impellers, the one that pumps the seawater which draws from the sea through a stopcock via a strainer normally(is it clear?) then through various heat exchangers before exiting via the exhaust. In the 6 cylinder Fords this is gear driven from memory, is yours driven by a belt? The other impeller is driven by the belt in your picture and pumps coolant round the engine and heat exchanger and is a closed system, same as a car, this system is normally reliable.
When was your seawater impeller last changed? Does the seawater flow out the exhaust look reasonable?
Last time I had an overheat (6 cylinder Ford) it was a faulty oil cooler, traced with the aid of a non contact thermometer.