If you take the average truck engine and drop it in a boat, it is usually tilted to align the shaft. This means the oil is now deeper at the back of the sump and now the crankshaft is immersed in oil. This leads to significant aeration of the oil and blow-by. Get one of the various recovery systems and filter the oil out of the discharge and return it to the sump.
Summit Racing is a good online source.
A common problem with Lehman engines is the marinizer forgot to re-mark the dipstick, resulting in too much oil being put in the pan with the incorrect markings; weeping seals and high oil consumption is a result. Many owners know their Lehmans will seek their own oil level and stay at that level until the next oil change.
At least once in your maintenance cycle, add the correct amount of oil as per the manufacturer, wait a half an hour and then look at the dipstick - is yours correct? You might need to remark yours.