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If one is running non-ethanol gas via topside tanks, do they need a water separator?
Thanks

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If the gas sits for a period of time. Always....

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Not a bad idea especially if you ever fill up at a dock...

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I see no reason to install a fuel water separator when using an on deck fuel tank. A waste of time and expense in this application.

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Gas gets contaminated in portable fuel tanks to.

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I see no reason not to use a filter/separator. Tank below deck, tank above deck, no difference.

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The original question cites the use of non-ethanol blended fuel in a topside fuel tank, and a concern of water infiltration. Non-ethanol fuel is not subject to the phase separation of ethanol blended fuels. Topside fuel tanks also do not produce the condensation of below deck metal fuel tanks. Your outboard engine contains a fuel filter under the cowl that will perform adequate filtering for engines operating non- ethanol blended fuels. Proper maintenance of this filter is more than adequate to contain any contaminants.

However, if you feel the need for an an additional external filter, your should check the capacity of your engines fuel pump to draw fuel through such a filter application, and and that the filter is designed for the fuel flow rate of your engine.

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Fuel injectors don't like water, or dirt ...

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No.

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A Fuel Water Separator is a relatively inexpensive way to avoid fuel problems irregardless of your fuel/tank type. When installed and maintained properly it will not contribute to increased chances of fuel starvation as some have suggested in the past.
Needed? No, but a good investment.