If we are talking Mercruiser, I have an older 1st gen Alpha drive (technically its an MR drive) and I have been using regular non-marine gear oil for the last 8-10 seasons or so. I know for a fact my friend uses non-marine gear oil in his too.
I cant really say I know what the difference in marine vs non-marine gear oil is, other than a 162% difference in price. An added water emulsifier would be my guess, but I dont really know.
I'm not recommending you should use non-marine oil in yours (quite the opposite, actually), I'm just throwing my own experience out there....I dont put a ton of hours on my boat, its a 4-banger, so not a lot of stress on the drive. A few hours of puttering around on the weekends during the summer months. So far, no problems.
Let me say that differently. If there is a problem, or if I'm in the process of creating one, it hasnt shown itself.....yet.