There is always a crankcase breather because there is always some blowby. Also, because the engine is not mounted horizontally, if the engine is used at higher power settings, the boat raises its bow and makes the engine tilt even further resulting in the rear of the crankshaft splashing in the sump oil and causing the oil to foam and vapourize. Imho, this tilt is the root of the #6 failure.
Also if the engine has never had its dipstick remarked the tendency is to fill the oil to the full mark which will be incorrect, way overfilled. That’s why you need to measure the oil you put in then remark the dipstick. If you don’t, the foaming etc will rapidly use up the extra oil and puts an old engine at risk of a runaway.
If you find your oil level in your Lehman drops precipitously after an oil change, then stabilizes, chances are great your dipstick is marked wrong.
Don't believe everything that you think.