One more point... Ethanol in fuel absorbs water. This is bad for the fuel system and for the whole engine.
Modern boats have water separator filters in the fuel system. My Volvo-Penta 5.7 GXi has a big filter in the fuel line, just before the fuel pump. It's about the size of an oil filter, costs $10, and lasts the entire season. These filters are there to separate any water from the fuel, and capture it before it even gets to the fuel pump.
You could fill the tank half full of Seafoam or any other fuel treatment, and it wouldn't "separate" 1% of the water that filter does. All the water would go right into the engine along with the fuel and the Seafoam. How does that help anything?
Ken