Just FYI -- Sta-Bil makes a "marine grade" product that is better for gas that contains ethanol. The Older engines were not designed with parts (gaskets etc.) that could take the ethanol. The newer engines are designed to take this fuel. Some of this post may be a repeat of what others have already said.
I've also read, and heard, about the marine grade Stabil and another product called StarTron. Some people swear by these products, but it's tough to find unbiased test results on them. StarTron claims to stabilize fuel for up to 2 years, cure ethanol problems, make the engine run smoothly, rejuvenate old fuel, improve fuel economy, and remove water. Plus more! They say they use an enzyme that alters the molecular structure of gasoline; beats me.
Here's a response from a StarTron VP to a complaint by a poster on a boating site:
Startron - Moderated Discussion Areas
I consider this to be a good example of overselling the benefits of a product.
All these products attempt to deal with excessive water in fuel by mixing it with the gasoline and running it through the motor. At some point, the amount of water being burned with the fuel will exceed the limit that the engine can safely handle. I would rather have my motor fail to start than to suffer severe damage. There's nothing being sold to consumers that will safely "remove water from gasoline", and once the water/alcohol mix has separated from the gasoline there's not much you're going to do about it except drain the tank. Products that make the claim that they will "fix" separated fuel often contain large amounts of alcohol, much like dry gas. Many fuel treatment products contain alcohol, which you may not want to add to the existing 10% ethanol already in the fuel. I use Seafoam, which is one of these. The primary ingredient in most treatment products is a form of petroleum distillates, commonly naphtha. Stabil and StarTron both use naphtha, while Stabil Marine uses a different petroleum distillate.
Best shot: Read the product label and remain skeptical.