I use SeaFoam fuel treatment in the gasoline fuel I store over winter in various engines, both 4 and 2 stroke, with no problems. [Caveat - none of these are outboard motors, tho I can't imagine any difference.] For stabilizing fuel, I notice the manufacturer recommends a mix rate of 2 to 4 times as much for 2-stroke fuel as for 4-stroke fuel.
My procedure is to mix the additive with the fuel as recommended, run the engine
long enough to be sure the carbs are full of treated fuel, and then top the tank up to eliminate air (and the moisture it carries). Six months or more later, and they start right up.
If you feel the need to fog
the internals, you can turn the engine over a few times with the ignition disconnected as a last step.
Is there any reason this wouldn't work with the ethanol/gasoline mix?