It is highly recommended that the following precautions be taken if you cannot find a source of ethanol-free gas and must use E-10.
First, if you do not already have an inline fuel filter between your fuel tank and your engine, it is strongly recommended that you install a 10-micron fuel filter with water-trapping capability. It should be installed in your fuel line before your engine, and at a location which is easily accessible for periodic change-out of the filter canister. These filters can be found at most boat dealers and Academy Sports + Outdoors, etc. The filter cartridge (it looks like an auto oil filter) should be changed every 100 hours of engine use or at least annually.
Secondly, it is strongly recommended that a fuel additive be used. I've researched at least four different fuel additives, and believe the "Sta-Bil" Marine Formula (blue color) is the better of the options available. This is because it has much higher levels of fuel stabilizer than others, and it also has a corrosion inhibitor that other additives do not have. The blue Sta-Bil is sold at most Wal-Mart stores and some boat dealers. A 32-ounce bottle sells for $19. 95 plus tax, but it only takes 1 ounce of additive to 10 gallons of gasoline. So a 32-ounce bottle will provide protection for 320 gallons of gas.
Part of the reason to use of a good fuel additive is not only to reduce the possibility of corrosion to your fuel tank, but also to stabilize the gasoline. Ethanol gasoline has a much shorter "shelf life" than gasoline without ethanol. The stable life of ethanol gasoline is only 2 months.
Lastly, keeping your boat's fuel tank full may reduce the amount of water that is introduced into the tank due to "breathing" since there is very little vapor space left in the tank when it is full. This has to be weighed against the poorer shelf life of ethanol gasoline.