The only boats (or cars) that are really at risk with E-10 are the ones that don’t get used. They absorb water, and then just sit. Once the gas has phase separated, and the ethanol is on the bottom, the remaining fuel is no longer hygroscopic. However, it will deteriorate beyond recovery if it sits for more than a couple of months without a proper additive, and corrosion will begin on metal components of the fuel system.