Since we are in dead of winter and not much boating going on. Figured I would ask this question now. Im sure there are many opinions.
I boat most every weekend late spring thru early fall. I run ethanol fuel during my boating season to save a few bucks. Later in the year I run it low and fill with non-ethanol. From then on I use non-ethanol so when I do stop boating it is full with non-ethanol and sta-bil. Motor is a merc 350 mag cool fuel non-cat motor. So does switching back and forth on fuel hurt or does it matter ?

I do the same...never a problem!

Never a problem here either. First tank of year is Ethanol as I fill up on the street. Most the rest of the summer after putting boat on the lift is non-Ethanol as that is what the marina carries. Will occasionally pull and fill up on street during season with Ethanol if the marina gets too insane on their gas prices. No fuel related issues in 13 seasons.

I can't find anything over 90 octane on the water so I've been buying premium with ethanol on the street for 10 years. I do run an oversized filter/water separator and I have typically drained the tank after every season. This year winter closed in before I could run the tank out. I have used StaBil in the past with very bad results, this year I am trying an Amsoil product. Otherwise no issues.

Startron is also a great stabilizer.