No good ideas that don't imply a major mechanical problem. It would have been nice if you had turned up a crankcase ventilation issue, but you have covered that. If the oil is leaving the crankcase and getting into the cylinders to burn it has three routes: induction air (you have ruled out), valve guides, or piston rings How much oil does it use? Have you changed anything recently? Why does the engine in a 2013 boat have only 35 hrs on it? If the engine sat for an extended period it may have had corrosion issues in the cylinders. They have bore scopes that can be used to look at the inside of aircraft engine cylinders. You might be able to remove the injectors and look at the cylinders through the holes. Look for scoring or corrosion.
Interesting puzzle - keep us posted.
Larry