The engine oil in your boats engine should be changed every 50-100 hours which is recommended by most manufactures.
Sometimes you might want to change the boat oil at least every year before you get ready to go boating depending on how often you go boating.
The more often you use your boat the more often you should change the boat oil and it's not gonna hurt anything if you change the oil too often.
As long as you're changing the boat oil every 100 hours of use then you're doing good.