Issue of oil change seems to be front and centre with all boaters. The mfg of engines sets the calendar to ensure that the engine will meet its forecasted life. The issue with moisture in oil, along with other contaminates do have an affect on a dormant engine. Considering that most boats are stored on the hard for upwards of six to seven months, that engine deserves fresh oil. As for those of us who leave our pride and joy afloat, once again many engines never are fired during the Nov-May period should follow the same advice. The engine left with dirty oil, and if diesel is the fuel the oil is full of carbon and that build can and will affect rings, valves as well as seals.
The curse to us all is the "dockstarter". He who comes to the boat and starts the engine at the dock. Lets it idle to show operating temperature then shuts her down. The oil has not really reached proper temperature notwithstanding the temp gauge reading. But what happens is you leave a warm block that as it cools creates moisture within. That moisture level will never show on an oil test but that H2O will attach itself to metal and other bits and pieces shortening their live.
Oil is inexpensive, systems to remove and replace are similarly inexpensive when compared to major engine repair. Any moderately mechanically adept boater can do the task. Follow the mfg advice, change annually, or when specific hours are attained, but nevertheless, an annual change should always be done. Set your maintenance calendar accordingly.