At least locally it is the water condensation on the inside of the tank that eventually will introduce enough water (it only takes a small amount) to bond with the ethanol and cause separation.
Here is an interesting video. Not sure about the product, but it illustrates the effect nonetheless.
https://www.youtube.com/watch?v=YeCyFxoWPpo
I agree most engines don't benefit from high octane - the issue is the Ethanol is hydrophilic. It will actually absorb water directly from moist air. Once water bonds with ethanol it forms an azeotropic mixture that cannot be broken down without drying agents that have a higher affinity for water than ethanol. I believe this is how some of the fuel additives work - the problem is it takes a large volume of additive to remove even a liter of water. Pure gas consists mainly of hydrophobic compounds.so it is typically less of an issue, but water condensation on the inside of the tank walls esp. when partially full will eventually cause an issue given enough time. I also worry when I use a pressure washer around the tank vent.
I am not an expert, but suspect agitating gasoline with ethanol actually makes this issue worse because it speeds up the kinetics for water absorption. The only way to prevent it is to burn it before enough water accumulates in the tank to cause phase separation. Ethanol can pull through small amounts of water, allowing it to be safely burned. If you are planing to burn the pumped fuel immediately you can probably get away with an ethanol blend in your boat if you are confident that there is no water in your fuel tank. If your boat sits with ethanol in it you are taking a big chance.
