The issue with E15 is the amount of ethanol content in the gas. Star Tron, from what I'm reading, is like Stabil so it's not going to do what you need it to do - reduce the ethanol content.
Rec gas on the hard or on the water look to be your options.
If you can get E10, you can use that but if you boat is older and never used E gas of any kind, expect it to clean out the gas tank and may clog filters. E10 is all we can get here and after the first year I didn't really have problems with it.
I don't know in what state you live, but here in Minnesota the street rod association maintains a list of stations where you can get non-oxygenated (Rec) gas. Do some searching and you might find some.
Gas stations near lakes or oceans tend to have recreational gas available, versus the one by your house.
I’ve run the boat (4.3 4bbl Quadrajet), 4 four stroke small engines and 5 two stroke engines and all have been run acceptably well on E10. However what annoys me is there really isn’t a choice. And there is really no benefit to the consumer of E10. None, at all.
I don't think there's any treatment that will help you with E15. With some newer boats using E15 voids the warranty. It doesn't say "unless you also add . . "
Why can't you find non-ethanol gas? Pure-gas.org lists close to 17,000 stations in the US and Canada that sell non-ethanol. Are you in Ohio? There are 249 listed in Ohio alone. I'm pretty fortunate here in VA, non-ethanol is readily available. Pure-gas.org lists 468 stations in VA.
