[jmk]

Yes, the rubber stoppers stop water that gets beyond your deck fill cover and prevents it from going further down into the fuel fill hose. Stopper MUST be large enough to never ever get pushed into the hose, so make sure it has a sufficient taper to prevent this. I grease it to make it easy to remove. I also put a screw into the top of the stopper so the deck fill cap pushes on the screw and holds the stopper firmly into the hole.

In a perfect world, the o-ring around your deck fill would stop water from coming in. But I have greased and even replaced that o-ring to no avail. The metal sealing surface needs to be sanded and polished to create a better fit, and you cannot do that easily on a tank fill without worrying about an explosion. Hence the cork. On a 36 Open the fuel fill cap is continually getting wet with run-off. So any small leak will put water in the tank. Perhaps something peculiar to the placement of the fill.