1 Does you boat have a built in gas tank or does it have a portable / removable gas tank?
2 Is your outboard engine a 2 stroke that you mix the oil with the gas?
reason for asking 1: Some boat makers made built-in tanks by forming the fiberglass in a way that produced a sealed chamber. They hooked a filler, vent and supply lines to these chambers and they became a fuel tank. Up until the 70s some manufactures did this. It was fine engineering at the time because the gasoline then was 100% petroleum and the fiberglass resin does not react with it. Now they put ethanol in the gasoline which, if it comes out of solution with the gasoline, will dissolve the resin. If your boat has a plastic or metal tank that the manufacturer built around or there's a portable one, then no problem. Just check your rubber fuel lines at least once a season. Eventually they will need to be replaced because the ethanol will soften them.
Reason for asking 2: Having gasoline with ethanol in it can ruin a 2 stroke engine in 3 ways.
First, The ethanol can come out of solution with the gas. Ethanol is heavier than gasoline so it sinks to the bottom of the tank. The oil that's mixed with the fuel stays in the gasoline. In an extreme case of ethanol separation there can be a pool of oil free ethanol at the bottom of the tank. If this pool is large enough for the engine to suck up the engine can quickly destroy itself. The best way to prevent this: If you plan on not using the boat for a long period, more than a few weeks, make sure the tank is almost full. For winter storage, make sure the tank is as empty as you can get it. Any ethanol that comes out of solution over the winter will be mixed back in when the tank is filled in the spring.
Second, ethanol has a very high affinity to absorb moisture. Weather its from humid air or from direct contact with water. The ethanol can absorb an amount of moisture and still stay in solution with the gasoline. It will go through the engine without much trouble but this moisture can cause metal parts in the fuel system to corrode. The best way to prevent this problem: StaBil has introduced a new fuel stabilizer that is blue and works just like the old stabilizer but it is made to be used with ethanol blended gasoline and help prevent fuel system corrosion that can happen with ethanol blended fuel. The second thing that can be done is add a water separator in the fuel system between the tank and the engine.
Third, The ethanol in the gasoline can react with the 2 stroke oil mist inside the engine. The process is called saponification, where an alcohol reacts with a hydrocarbon in the presence of heat and agitation to turn into soap. There is no way to stop this process other than using leaded aviation gasoline or unleaded racing gasoline which both don't have ethanol but are hard to get and cost up to $9 a gallon. But what can be done is to make sure that there is enough oil mixed with the gas so that after this process happens there is still enough oil to lubricate the engine. This is a problem that is more likely to happen in an air cooled 2 stroke engine that runs hot like a dirt bike or chain saw. I figured I'd mention it anyways. For example, if your outboard uses a 50:1 fuel oil mix thats about 2.6 ounces per gallon. If you used 3 ounces per gallon then you'd never have a problem.
