I too learned the hard way. Bought my '78 RANGER just before Thanksgiving and just had to get out on the water forgoing any precautionary checkups.
While setting the hook in a secluded cove, the engine temp rapidly rose and zero water was exiting the exhaust.
I knew instantly it was the impeller.
The bright side of that dark cloud is that it sent me searching out info on this (new to me) little engine and I found this extraordinary A4 site! Since then I've replaced the water pump, thermostat & housing, hoses and exhaust hot section. Thank you Don (and all contributors) for this most wonderful resource!
Anyway, back on topic...
I plan on checking mine at least every 50 hours and religiously
(or maybe prudently? knitchie) following
rigspelt's 5 rules of thumb for replacing.