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).
http://www.boatsandrealty.com/boats/CINDEX.HTM
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).