Well, this is a really interesting new issue. A first on this forum, I would think?
I ran a big dory for several years, with an older two stroke Mercury 60hp on it. You're right, I wouldn't want to feed it Aspen- ££££! Mine was carb fed. As that boat lived on a mooring, and we had (still have) no pumped petrol in Fowey, I fuelled mine by taking the big 25 ltr tanks to the garage. A pal with a big ski boat found it easier to haul the boat out, and take the whole thing to the garage, as he had an underfloor tanks of about 50 gallons!
Hopefully, yours will be fuel injected? Better for fuel consumption , emissions etc.
So, What does OEM say re E10 fuel?
If injected, my guess - as I have no experience- is that you should be ok if you try to avoid/minimise cheap and nasty fuel. Marina petrol is often not the best anyways?
The issue seems to be with this new crappy petrol in small carburated engines, as i can run our petrol car successfully on fuel that cannot be used in my small petrol outboards.
Similarly, but different. My garden machines run fine on reject petrol.
They are 2 stroke and so have more tolerant carb settings?
If a fuel injected outboard, i think you'll be ok.
Looking forward to learning more about this!