I use E10, mainly because my engine is designed to use it, and I do a lot of Nm, so it is a bit cheaper. Also, I use my boat all year round, there is some good fishing to be had in the winter months !
However, I also treat the fuel with Mercury Quickstor, just in case the boat is laid up for any length of time. Quickstor maintains the octane and also stops the Ethanol from separating in the fuel. Another advantage is its lubrication properties for the fuel system and injectors.
With E5 in your tank after just a few months, then just get on and use it, the engine will be fine