IMHO anyone buying a boat with a marine diesel should pay an engine surveyor to survey the engine, for the same reasons that you have a marine surveyor survey the boat itself. Like the boat survey, it is $250 (or whatever) very well spent. A reliable picture of an''s engine true health cannot be obtained from a one-time observation. Include a compression test.
The one thing a seller will not tell a buyer is anything the buyer really needs to know!
As an example, during our last boat purchase we had a mechanic test drive the engine prior to purchase, and he gave it an ok evaluation. We ended up with a full engine rebuild at the end of the first season, as difficulty in cold weather starting demonstrated low compression. This was a $10,000 mistake.
As the buy guys say "trust, but verify".