As I recall Octane is a number on a sale that relates pressure to pre-ignition, or pinging as you used to get in auto engines. Low compression engines can run on low octane fuel. Back in the hot rod race of the '70's moving up the compression scale from the lower compression (commonly used) single digits to something like 12:1, like the '72 Chev. 409 2 door hardtop hot rod (numbers may be a tad off....been a long time), required a premium, higher octane fuel to keep the fuel mixture in the compressing cylinder from exploding (self igniting) prior to the spark application.
Your 115 is not a high compression engine!
If you don't want to run ethanol and choose to pay the premium price, and premium is the only non-ethanol fuel in your area, run your premium. One thing to think about and that is how old is the gas at the service station? I don't know what the storage life of fuel is in underground storage at service stations, but if you look at the fuel select buttons on fuel pumps, you'll find the 87 button well worn and the 91 or 2 or 3...whatever it is these days, shiny and new.
I had a new '88 version of that engine back in "89 and ran 87 unleaded all the time and did so for 7ish years I had it. It probably didn't see ethanol, forget the timing of these things, but I didn't use any special fuel. I had the right boat, right prop, and it ran like a scalded ape. I loved it.
