I agree that maintenance is key. However, that is unknowable here, unless the P.O. kept a maintenance log. I have a 35 year old Atomic 4 (RWC), rebuilt in 2005 and "treated" with a post-impeller, pre-block filter basket that in my mind keeps more crap out of the block that might cause issues. I also do a muriatic acid flush every four years or so, but I rarely find much in the way of corrosion.
To this I credit the fresh water of Lake Ontario and my 180F thermostat.
Seawater cooling is inherently trickier, of course, because you have to run the engine cooler than is ideal because of precipitating salts at 155 F (if I recall). An engine flush and a good look at the hose barbs will reveal much here.