According to boat and motor manufacturers, 20 hours is the point at which most boats and engines are properly "broken in" and any problems caused by friction or break-in will have occurred by this point. The engine can also use more oil during this period so checking the oil level and changing the oil is important. This is the best time to perform an inspection on all elements of the boat and motor.
-check oil level and quality, check engine mounting and gel-coat quality, check engine cooling
-replace oil, oil filter, fuel filters, gear lube, verify propeller alignment and tightness, skeg, and anodes