how much is dictated by seasons up here (in Canada). Since boats come out of the water
in Oct/Nov and consequently engines are decommissioned for winter storage
, often oil and filter changes are combined with the recommissioning in the Spring (or the decommissionning in the Fall). These are recreational sailboats, however, who do not put on a significant amount of engine hours seasonally.
Impellers are checked regularly by mechanics (again with seasonal de/re-commissioning), but only changed when they show wear or when the owner calls us with overheating
In an ideal world, boat owners would follow the manufacturer's specs, but overall, it does tend to fall to the individual boat owner's inclination to keep up their engine maintenance
(or not) as the case may be.
That's just what I've seen.
Good luck with your new boat! Wish I was motoring/sailing through the ICW
right about now!