Some change it at less hours than recommended by the engine manufacturer. Some change it at slightly more. Some change it at convenient stops when cruising and worry less anout the hours. Some change it based on looks. Some change it based on oil sampling. Some change it based on what they hear from other boaters or mechanics that change it based on what they have heard from othe boaters or mechanics.
Hardle anyone in the group can cite a major engine problem based solely on engine oil change frequency.
Many when discussing the subject never really include the variables such as engine make, age, usage, application, etc.
Then others will tell you it depends on brand and oil type.
So bottom line is..... who do you trust for the best info? Or the info that best applies to your situation? Or even should your change frequency be the same based on your usage?
Maybe thats why your mech suggested a different time than the manufacturer. Even in cars they recommend based on normal or hard usage.
You will get plenty of opinions for sure.......