Can speak to my experience with my 3GMC raw water
cooled, bought new in 1985, have for 27 years. Critics say motor
should have rusted through years ago, my Yanmar
we will both be long gone and motor
will be going strong.
I flush motor with Rydlyme every few years and change the 3
motor zincs,(usually do not need to be changed) again every few years. Run fresh water thru and use rust inhibit anti-freeze at end of season. Took exhaust
manifold off this season just to take a look, was clean and open.
If a raw water motor is allowed to overheat, the salt
will separate and line the water passages inhibiting cooling...so flushing
is important. That is why raw water motors runs cooler
(with a 140 degree thermostat) than a fresh water cooled motor.
Hence your hot water will be hotter with the fresh water motor.
One must flush/clean heat exchanger
on fresh water system also, some remove and bring to auto radiator shop to clean.
There are no zincs in the Yanmar fresh water motors.
All in all most prefer fresh water cooled.