Eric,
Sorry, but I'm going to have to throw a flag on that statement. The Volvo MD series marine diesels were designed and built for one thing, and one thing only, boats. Granted, later small marine diesels are often based on industrial or automotive designs, but some engines are definitely marine only.
That might be a purpose built marine engine, but geez it is sea water cooled. I've spent a bunch of time trying to chip out corrosion blocking coolant channels on those. Most aggravating. Direct cooling is a definite cheap-out.
It does not matter if an engine is built for marine, automotive, industrial, agriculture, whatever. An engine just converts fuel and air into spin, heat, noise and smoke. Does not matter what it started life as, they all do the same thing. We just want the "spin".
What matters is the quality of the engineering that adapts the engine to the unique marine environment.
A few rules seem to apply:
NO ALUMINUM in contact with coolant. Volvo and Yanmar fail here. Lots of aluminum here like whole exhaust manifolds. So what, save 20lbs, create maintenance nightmare. Detroit, Cat and Cummins- Zero Al in contact with coolant.
No sea water oil coolers. Volvo and Yanmar again fail. Detroit, Cat and Cummins simply do not do that. Coolant cools oil, much better temp control, no corrosion issues. Oil hot enough to keep dry.
Simple serviceable coolers. Cummins uses Serck and Sen-Dure coolers, easy to service and easy to remove from engine. Cat and Yanmar similar, easy to service. Some Volvo stuff is a real PITA and since made in house, parts availability is up to them.
I can get a 30yr old Detroit cooler direct from Sen-Dure.
There are a few valid complaints about the Cummins- Stupid leaking sherwood pumps and having to pull a mount to replace. Putting the leaky pump above the HPFP on the QSB...
But in general, Cummins does a better job than just about anyone else.
I don't have much Deere experience, just not many in my fleet. Generally very good as I hear, but I think they use aluminum in coolant path too. Someone else might confirm..
Still waiting on stupid building permits. This is killing me.