[MOTORHEAD]

John; If it was only run in fresh water, or has a raw water/anti-freeze heat exchanger, it should be worth checking further. If it has been run in salt water with out a fresh water cooling system installed since new, forget it !!!!
If its a block thats never been in salt water, tear it down and have your local engine shop pull all freeze plugs and do a visual inspection of the water jackets. ( if they have a borescope, even better!) then they can pressure ck it for leaks/cracks.
There are other tests that can be done ( sonic, x-ray,) but usually
a visual and pressure test is where to start.
But also be aware that marine engines lead a much tougher life than any car engine!!! Boats can't get up to speed and back off the throttle and still maintain that speed like a car can. A boat engine is always going"uphill"!! So be on the lookout for badly worn cylinders and stress cracks in the block.
"Be careful out there"
Ted