My manual states "Use gasoline with a minimum octane rating of 89."

Also, my old IO stated something similar (I'm sure the words it used were "at least 89 octane"), right on the throttle body cover when you open the dog house.

But hey, I guess I could save some money going down to 89. Your right - running 91 won't result in any more power in the boat - maybe I got sucked in by marketing saying that it just might be cleaner burning or something.