Bet they had beefier castings then. Can't you just order a Yanmar with no heat exchanger and run sea water directly through the engine? The last salt water cooled engine I had was a Chrysler Crown L head
These were prone to getting porous over time from salt water. There is some sort of reaction other than oxidation that seems to take place over time between cast iron and NaCl water.
I don't think you can order the engine without heat exchanger. I could buy it and take it off myself, but I do not believe the engine would last long as it probably does not have the zinc installed in the block and head (like a 3gmd does), and the iron is probably thinner and would corrode through faster.
I think what happens to cast iron in sea water is that the iron corrodes away leaving only the graphite- which has little strength and crumbles easily. This is known as graphitization or graphite corrosion. The iron matrix corrodes, leaving behind a porous graphite that has no strength. Significant weakening can occur without much dimensional change.