First of all, these oil discussions, whether they pertain to boats, cars, or anything else, are always more speculation and "this way makes me feel better so it must be better" than anything to do with established fact. So, each person must decide which version of the speculation they subscribe to.
Me, I subscribe to the following:
1. My manual says to use 30w synthetic. I'd be using synthetic in any case, and since the VP oil isn't much more $$ than M1 if you know where to buy it, what possible reason could I have to not use it? I believe as strongly in the quality of VP oil as I do in M1.
2. Marine engine oil *is* subject to far more damaging loads than almost any auto engine. I don't know about anyone else, but if I ran my truck at 4,500-5,000 RPM's for more than a *very* short period, it'd be toast (it's M1 5W30 notwithstanding). Not only do we do this to our boat engines with regularity, we follow that with sitting in storage for weeks or longer at a time - and then run it at 5 grand again. I subscribe to the idea that the viscosity packages used in mult-weights may very well have a problem holding up in these conditions. And since I'm not boating in cold weather, why would I even *want* a multi?
3. I also subscribe to the idea that marine oil is better at holding off the effects of moisture in the oil. When your auto engine is running at normal temperature, the coolant is flowing through the engine at a similar temp - typically around 180F. When the "coolant" flows through some parts of your boat engine at some times, it's what - 70-80F or less? Applying this colder water to a hot iron surface, in my mind, *has* to result in the chance of some amount of moisture forming internally.
It's the *combination* of these reasons that cause me to stick to the VP oil, even though I'm the world's biggest believer in M1 for everything else.
And now you know why I try to just stay out of oil discussions
Again, this is what *I* subscribe to, and what others subscribe to may be every bit (if not more) valid (as far as *any* of us know).