This is the time of the year that boat owners need to be more aware of the problem with ethanol fuel additives. Any gasline that has been blended with ethanol has a shelf life of about 60 days. This means that if you have an ethanol blend in your boat fuel tank, and you do not use the boat for two month, you will have a big problem when you try to use the boat again. The ethanol in the fuel absorbes water through the venting system on your boat. In some older fuel tanks, it will even begin to eat away at the resins that hold the fiberglass together, eventually ruining your tank. Since boats are normally on the water where moisture content in the air is at its highest, significant amounts of water can be absorbed in a relatively short time. The photo below shows fuel that I took from one of my tanks. You can clearly see that more than 20% of the liquid in the bottle has turned to a waste mixture. What is not so clear in this photo is the thick layer of gunk, between the water layer and the fuel. This is from the disolved resin on my fuel tank.
There is no solution for this problem, only preventive measures. Once water has gathered in your tank, there is no additive that will remove it. if you find that this has happened, you most drain the tank and get rid of the bad fuel. As a preventive measure you can still buy fuel without ethanol added in some marinas and at some gas stations. This is your best bet. If you use ethanol fuel, keep your tanks topped off at all times after every use. If you want to use an additive, make sure you use one that has no alcohol in it, there are very few of this type but there are some. Also make sure that you add the additive when you add fuel, if you add it afterwards, it will do no good.