^^^ This ^^^... It's about the worry of salt crystallizing in the cooling passages.
and ... its important to remember, water temperatures are not oil temperatures. Marine oil temperatures run far hotter then the water temperature.
This goes for any vehicle, but way more so in marine engines that run under incredible stress pushing a boat though water vs a vehicle with wheels.
Throw in the high RPMs, stress factor, bearing heat, super hot pistons getting cooled with oil being sprayed on them and I am sure your oil is much hotter then a automoble running at a higher water temperature then a boat.
Plus its in an enclosed space. Water temperature is not oil temperature. I think many would be surprised if they check oil temperature.
Two different fluids, doesnt make sense to talk about the temperature of oil by taking readings of the temperature of the water anymore then it would to talk about the temperature of the water by taking an oil temperature reading.
BTW my Volvo engine is spec'd for 20w50 oil