Always, always, get your fuel at the marina. Not the local convenience store, rez, etc. The fuel at the marina is formulated without ethanol, which is what is eating gaskets, causing detonation, etc. It also has a lead content, yes lead, which greatly helps to lubricate valves. The additives you are buying for your store bought gas will not remove the ethanol (it is 10-15% of each gallon), so don't waste your money. The additives simply help to provide a barrier between the ethanol and those parts that can be harmed.
Just get your fuel at the local marina. Your boat motor will thank you. Add sta-bil in the fall, you will be trouble free for years to come.
Try it in your weed wackers, chain saws, lawnmowers, and anything else with a small engine. You will be amazed at how much better they run.
And no, I don't own the gas dock at the marina......