A petrol (US gasoline) engine will probably have a usefull
life of a few thousand hours, OTOH decent diesels such as
CAT or Volvo should be good for between 10,000 and 20,000
hours because they are designed for heavy truck duty.


Dream on. The only way to realize 10,000 - 20,000 or even many more
hours from a marine diesel would be to run it almost continuously. The
average intermittent use pleasure boater will be very lucky to get
5,000 hours from a diesel, and even then it will need to be one of the
slower turning models.
You even find some old trawlers with hours up in the 6-7K range, but
10,000 or more is almost unheard of. Cripes, it would take the average
pleasure boater 50-100 years to do 10,000 hours and by that time the
darn thing would simply rust apart. :-)