I wouldn't be put off by salt water
cooling on the Volvo MD series of engines. They were designed for salt water
cooling and didn't seem to have problems in reliability
life. The MD2 in our old boat went on for more than 20 years, maybe 30 and 3 trips to SoPac before the then owner replaced it. Don't know what finally went wrong with it but suspect it was wanting more hp had a big factor in the decision to replace rather than rebuild
It is probably not a factor in choosing an MD series engine now because of parts availability and cost, but every part that might suffer from cooling water corrosion
was replaceable. Further, you could totally rebuild
the engines, with maybe the exception of the main bearings, with the engine in the boat. Rod bearings, cylinders, heads etc were all removable without having to pull the engine. I managed to hydrolock my engine when new that shortened a connecting rod. The mech. pulled the offending cylinder's head
, cylinder, piston, changed out the rod and put in new bearings with the boat in it's slip.