The key is maintenance, clean fuel
, & don''t run them full throttle(about 80% of max rpm is an upper limit for longer term running). Yachtworld has 4 Whitby 42''s having 3500, 5000, 4000, & 2800 hours respectively. A few months back there were two more, one with 6000 hours & the other with 9000 hours.
There is lots of commercial Cape Island powerboats(Downeast-like) in the link below with 5,000-12,000 hours. They are commercials, but many of them have fairly low power (160hp or less).
All the diesels discussed here are naturally aspirated. Sportfishers running turbocharged, aftercooled, intercooled..... diesels have a short life...maybe 1000-1500 hours, then rebuilds run BIG bucks.
I wouldn''t eliminate a sailboat/diesel just due to a few hundred hours, but I would check it out well....as I would an older engine with 100 hours(less hours can be more problematic than high hours).