At the end of the season or last tank of fuel for the season I will use a pint to.my 18 gallon tank. Thru the season, so just add a few ounces to each filling ( about 12-13 gallons typically) . SeaFoam, Lucas fuel treatment, and Startron have all worked well... I use them to keep the fuel and combustion chambers clean from buildup. Before the E10 fuels came out, we did the same thing with "gumout" carb cleaner, just added a couple oz. to each filling to keep things working well. People often ask me why my boat engines run so smooth at idle, even for longer periods of time at very low RPM. Some folks call these snake oils and other names, but for me, they make a difference. In my 60 yrs of boating, never a fuel gunk issue, and my motors run so smooth, I find it well worth the pennies it costs to add.
I always buy used boats and motors and first thing to do is flush out all the gunk from the fuel/ carb systems. If needed rebuild carbs with factory kits. After getting everything adjusted properly, the added maintenance additives keep me from having to reclean things again down the road.