The fuel is for a car, not a boat. I know some marinas sell fuel with an average or maximum of 10% ethanol, whereas many or most service stations sell fuel with an average or at least 10%. However, I don’t think I can fuel my car at a marina. 
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ummmm, why?
This is truly a hard question! Do you want gas with less ethanol for a boat? If so you might look at the marinas. Many outboards don’t like ethanol.
You won’t get any different horsepower from gasoline and gasoline/ethanol mixtures of the same octane on a turbo car, or any other car.
Speaking of turbo cars and octane, ethanol is much more resistant to detonation than gasoline.
Contary to a another post, you DO get lower mileage with an ethanol blend because of its lower “heat content”.
There are test kits of alcohol in fuel, here’s one: About $4/test.
You could do you own water extraction, like in the test kit. But you won’t come out ahead economically, because you’ll be throwing away the alcohol (it is denatured with methanol and it will have carcinogenci benzene too, so DON’T drink it).
Check different retailers, they’ll get their gasoline from two or three different refineries in your area. Go to the local refinery or bulk fuel terminal and be amused to see 5 to 8 differently branded fuel trucks pull up. All those companies who like to say their gasoline is somehow better all get it from the some source.
You can always go to your local airport. 100LL avgas (100-octane low-lead aviation gasoline) definitely does not have alcohol in it because some bladders and fuel system comonents in planes aren’t compatible and engine failures and fuel leaks in an aircraft are taken VERY seriously by pilots. But it is a fair bit more expensive that “mogas” – regular vehicle fuel.