...unless, of course, you keep your boat
in the water (who would ever do THAT?) where the ethanol picks up the water from the dew that forms in the tank every night (or just the very humid layer of air that lies on the surface of a body of water, right about where your fuel tank vent is) until the ethanol is saturated and falls out of solution (phase separation) in your fuel tank.
Then, what you have left is a layer of very low octane gasoline (because ethanol is what boosted octane after MTBE was banned) floating on top of a layer of alcohol, water, rust and trash. Yes, trash – because ethanol is a great solvent for that buildup of gasoline varnish that has been accumulating in your tanks over the last umpteen years.
So if you're lucky enough to have your fuel pickups higher than the alcohol-water-rust-trash solution at the bottom of your tanks, your motors will at best run kinda sorta in "limp" mode because your knock sensors detect the problems caused by the low-octane fuel ... but watch out, because boats roll, pitch and yaw, and then you'll pick up some of that water-alcohol-rust-trash "maggot stew," which will swell and plug your fuel filters and rust components of your fuel system...
Yep, boats do just fine on E10! Got a new battery in your EPIRB?