If you have the tools (hand tools, only the required torque wrench is a bit exotic) and experience (or mechanical skills and can read a technical manual), I would first pull the valve cover and make sure the valves/springs/rockers/push rods are all doing there jobs and look good. I would then take the head off it and inspect the pistons, rings and cylinders. If it all looks okay, you can pull the oil pan to check for wear products, and inspect the bottom end of the engine. Neither expensive nor difficult to do these inspections. Don't know about your engine arrangement in your boat, but perhaps you can do the top end inspections without pulling the engine.