Since this will be a single outboard and fishing 15+ miles off shore, obviously the fuel reliability is an issue. Additionally, I've heard comments about the ethanol eating at engine parts too, though I need to gather more information on this aspect.
From what I've learned, many boats use aluminum tanks, but I'm surprised at how many dealers have no idea what kind of tanks are in the used boats on their lots. Almost makes me want to buy new, or at least new old stock, so that I'm dealing with the manufacturer rather than bob the plaid-clad sales rep.
Maybe if I wait to buy until next spring, I can buy directly from the United Boat Workers union, who, under obamacare, might end up owning all the manufacturing companies.
The ethanol eates away the rubber/plastic fuel lines in all motors(chain saw, trimmers,boat,etc.)
With 2 cycle engines the oil that you mix has a fuel stabilizer in it. With ALL other small four cycle engines
you should add fuel stabalizer to the gasoline/ethanol mix.
FYI, ALL offshore motor boats should have 2 engines.
You mentioned aluminum tanks, and then engine parts.
Ethanol is corrosive to soft metals such as zinc, magnesium, aluminum, etc. Any metal made of this must be plated (in chrome or electroless nickel), or must be anodized.