The basic problem with the GM V-8 car diesel that I referred to earlier (I don't know the displacement) is that it was created from a gasoline engine. According to the engine folks I have heard talk about this engine, this was the wrong way to go. The higher compression ratio of the diesel version was simply too much for the block and head, and over time the engine begins to fail in a variety of interesting ways. As I recall it was used in things like Cadillacs and Buicks and was an attempt to get better mileage from the big, heavy cars of the day.
So it was a bad concept to begin with.
Then somebody (I don't know who, perhaps GM themselves?) decided to marinize it. I have no idea how good or bad the marinization kit was. The problems I heard about the marine version of the engine were the same as I'd heard about with the vehicle version of the engine--- early failures due to the original design not holding up under the pressures of a diesel.
Other companies, like Land Rover, went the other way and created a gasoline engine from an existing diesel engine. This worked great because the gasoline engine benefited from the very robust design of the diesel engine. In the case of Land Rover, this was done so they only needed one engine assembly line instead of two.
But even if the boat in question has this iffy marinized car diesel in it, if the engines are currently sound and run well, there's little point in changing them out prematurely in my opinion. Run them until they don't and then deal with it. That could turn out to be many years of trouble-free use.
