Well, you won't hear me say "install a w/s filter" on any outboard not factory equipped with one. On older carb motors all filtering should be done between the fuel pump and carbs....with no restrictions whatsoever between the tank and fuel pump (with an exception to pontoon applications where the fuel tank location is ABOVE the outboard, in which case an anti-siphon valve is required for safety sake).
I have seen some issues that were truly caused from excessive alcohol in the fuel but in all honesty I think the worst cases I have dealt with were not from E10 pump fuel but rather from the witches brew of additives that many have been "sold on" and were using religiously.
Your outboard will be converted to all alcohol resistant seals, gaskets, and diaphragms and you won't have to install any add-on filtering components or use any additional additives whatsoever.
Bottom line is this: Old or bad fuel is "old bad fuel" (period), whether you let it sit too long...or whether it was alteady bad when you bought it.... and no filtering/separating, or commercial additives will refresh it or keep it from getting "old and bad". There is a big misconception (hype) about "fuel stabilizers" like Stabil and what they are designed to do that I'm not in the mood to write a book on right now, but I don't recommend their use at all.