When in doubt I always reach for a 15W40 in my Chev engines.
But...I bet that the engine oil operates at far higher temperatures in the hull of a boat than in a car, truck or tractor trailer.
How do VI improvers work when the entire lubricant is at a very high temperature is anyones guess.
Most industrial engine applications use mono-grades.
There are some very good SAE 30s and 40s of late with high VIs, such as Mobil's 1640 and 1630.
These oils will flow far better when cold than the old 100 VI offerings.
I look for flash points in the 250-270C range.
My rule of thumb is high flash points for hot operation, high VI for cold pumping and flow engine oil performance.
The flash point is the temperature where an oil is producing enough vapor to sustain an open flame.
Volatility, is the evaporation tendency of an engine oil.
It seems that when VI goes up as in multi-grade engine oils, the volatility goes up, and the flash point goes down, making those lubricants less suited for extremely hot operating temperatures.