High octane gas is more "flammable" than regular gas.
Outboards perform better with high octane gas.
You'll observe less "gunk" residue oozing from the exhaust.
Most engines perform fine with regular gas, however one of the issues is with Ethanol.
Ethanol is hygroscopic. This means it will attract water.
Water will corrode aluminum, especially in carburetors, even moreso if it's left stagnant inside your engine.
If you have a small outboard, an intelligent solution would be to have a smaller separate tank of ethanol free gas.
At the very end of every trip, switch tanks via a hose connection and run ethanol free high octane gas for a few final minutes. Disconnect the hose and run the outboard completely dry before you shut it down.
This will help you save a bit of money because regular gas is cheaper. If any fuel is left in your outboard it will be ethanol free high octane gas which is less detrimental to your engine.