If you are like me, you spill oil every time you add oil.
With the plug in, pour in a couple of cups of Simple Green. Then pour in a gallon of water. Then go fishing for the day.
The riding around in the boat sloshes Simple Green everywhere inside the hull where the oil has been going.
Pull the plug at the ramp parking lot before you leave and drive home. (Do this in a reasonable way, don't let the oil just pour back into the lake.)