Hey everyone,
Sorry we are late to the game here. We are here to help clear up a little confusion.
Your concerns about water in the tank are completely valid. Phase separation can actually occur when as little as .005% (4 teaspoons) water per gallon of fuel is absorbed. So if you had 20% of the tank full of water, you would be well past that point. Once phase separation has occurred, NO ADDITIVE in a reasonable amount is going to help fix that problem. You now have a water/ethanol mixture at the bottom of the tank that has an aggressive pH and gasoline sitting on top of that which has a reduced octane rating. You'll need to drain the tank and start over.
Good practice is to try and keep the tank as full as possible and use a high quality additive. This will help cut down room for condensation to form and the additive should have water handling capabilities, cleaning properties, and a rust & corrosion inhibitor.
For those that have not used additives and had success, keep doing what you have been doing. The fact is there are additives out there today that do help alleviate some issues with gasoline today (ethanol blended or not). It is important to do a good bit of research before you run any additive. There are some out there that make some pretty far reaching claims. There are others out there that are trusted and recommended by OEMs. Just be sure to do a little homework before dumping an additive in the tank.
Please let us know if you have any questions about fuel.
Have a great day!