There's been quite a bit of discussion on this subject over on BASH. According to the so called experts over there it seems that everything will be okay as long as your boat is made after 1988. Personally, I think it would be a good idea to change all your fuel filters before running ethanol in your boat.
Im with Mike. Im not too worried about it. The manual for my Merc says up to 15% is ok, in my state they are doing 10%. Really at this point its either live with it or dont use my boat since every gas station in my state is required to carry this blend.
I have a Merc 225 EFI. Called Mercury and they said that you can run up to 15% ethanol.
Plus, my EFI already has a water seperator filter on it. They said as a preventative, you might want to add stabil to your tank to help take more water out of the gas. Good Fish'n!