Black oil, hmmm,
I have put some thought to it and do have some thoughts. First of all, I think boats run rich as a rule. You cannot help getting carbon particulates in your oil with a rich burning engine. Second, Have you seen the air filter? Really small and more like a flame arrestor than anything else, plus its getting all its fresh air from the basically closed engine box. Third, high RPM most of the time, and dead idle the rest, not alot of mid level engine action. Fourth, lots of run the socks off then let it sit a couple weeks. I think this all would be considered severe conditions. It would be interesting to get some more folks to do an oil analysis at the end of the season. There is no telling what is in there after a tough summer on the lake.
Just some thoughts from the local "Aging Hipster"