Engine hours mean no more, and no less than "low miles" on an
automobile.

There's a chance that a boat that has 300-400 hours on it now will
eventually outlast a boat that has only 100 hours or so......

I'd be skepitcal of a 5 or 6 year old boat with only 100 hours on it,
unless you live in a climate with a *very* short season. A boat used
only 20 hours a year is probably a very low priority item in somebody's
life, and less likely to be maintained in some important ways that have
little to do with the engine. (Has the same oil been rotting in the
crankcase since 2000?)
I'd look just as thoroughly at a 5 year old engine with 100 hours as at
a 5 year old engine with 400-500 hours (more normal usage). There's
also a chance that the hour meter was disconnected (or replaced) since
the boat was new and you could be dealing with more hours than you
think. Unlike odometer tampering, it isn't a federal crime to swap out
hour meters, etc, on a boat.

Most gas engine boats will see 1000 hours of service, with even minimal
maintenance (which is what most of them seem to get) and excluding some
random, catastrophic event. A good portion will still be running
without a rebuild at 1500 hours, and some very lucky boaters with
stringent maintenance practices will realize 2000 hours or more before
rebuild or replacement is required.

It isn't entirely accurate, but maybe useful for illustration, to view
1000 hours, 1500 hours, and 2000 hours much like 100,000 mi, 150,000
mi, and 200,000 miles on the family auto engine.

In actuality, you boat engine sees the same service that an auto engine
would experience running about 70 mph, uphill, at all times.