Besides the emission reduction hoax of ethanol. Ethanol has less BTU, so it packes less of a punch per combustion event. This means less mileage/hours per gal. So more fuel used to do the same amount of work = hoax.

Ethanol is more corrosive then pure gasoline and is more prone to attracting moisture. With newer boats, like in the past 20 years, the corrosion part should not be an issue. Ethanol tolerant parts have been in wide use for years. The later however, CAN be an issue for some that have long layups or dont put many hours on in a season.

Personally, I do not worry much about ethanol. Use a quality layup fuel additive, regular maintenance, and you should not ever see an issue from ethanol use. Most important to me, is making sure im using the proper octane (or higher if thats whats available) level required. Ill take ethanol over engine damage as a result of detonation from using too low of an octane. And yes, I know what a knock sensor is, what they are for and how they work. Id rather avoid a situation where its called into action, if I can avoid it by just using the proper octane to begin with.