SpitfireS

When we're talking about marine engine oil I agree and you (Indy & Bror) have a good point.
The first thing Honda mentions in the advantages over automotive oil is the
Another thing about the "Honda Marine" oils, IMO most of the time Honda Marine is just a branch, like Honda Automotive or Honda Power Products.

When it comes to lower gear cases in outboards, I really don't think Honda expects water to enter the gear case and design the gear oil accordingly, not any more that it would in a diff under a car driven in rain.
Sure, its submerged, but not much.
Water pressure is low.
The change in temperature is more of a problem, just as it is 4X4'ing: you enter water with a hot diff, it cooles down and sucks in air past the seals if your breathers are blocked or too short.
There are special long diff breathers in the 4x4 world - for a reason.
I'm sure Honda engineers created a good breather system in outboards.

Compared to most automotive 75W-90 diff water oils I would take Honda Marine single grade SAE 90 GL-5 any day.
In an OEM diff that is.