Is it safe to run ethanol blended gasoline in your boat?
Many experts won't say an absolute "yes" or "no." However, they do offer several cautions that make the average boater avoid so called E-10 gas (ie. gas with 10% ethanol) like the plague.
In late 2010, the U.S. Environmental Protection Agency approved E-15 gas (Gasoline with 15% ethanol) for newer cars (More). It is unclear if that means we will begin to see the E-15 mix at gas stations and marinas around Lake Gaston. So far, we haven't.
So, back to the question of E-10. Should you run it in your boat?
Boat moter manufacturer Evinrude-Johnson says MORE than 10% " will cause performance problems, and can also cause permanent damage to your marine motor." (More)
But at even 10%, the company says there are three reasons you "may not" want to use E-10 gas.
Boaters often store gas in tanks longer than recommended for E10 (90 days). Cars, unlike boats, usually replace fuel every week or two, which will successfully prevent the possibility of water-contamination/phase separation.
Boat engines live in a water environment. Alcohol gas loves to absorb water. Ethanol E10 gas can absorb large amounts of water into the fuel tank, MTBE in conventional gasoline did not..
Boat engines usually last longer than cars. Still owning and using a marine engine from the 1970's or 1980's is not uncommon. These older engine parts and tanks were not usually designed or tested to withstand the damaging effects of alcohol gas. Several older marine engines (made prior to 1992) have plastic and rubber parts, and fiberglass tanks that are NOT compatible with E10 alcohol fuel.
And Evinrude-Johnson is among the most accepting of boat motor makers. For example Pleasure-Craft, Suziki and Nissan do not "reccomend" the use of fuel with any percentage of ethanol. (More)
And if you have an older boat motor, built prior to 2003, there is a good chance ethanol, even at only 10 percent, can cause problems. The older your engine, the more likely you'll see those problems.
Engine manufacturer Mercury says their newer moters "can run on 10% ethanol," but they note (almost word for word) the same reasons listed above by Evinrude-Johnson that you may not want to do that. And they note that running E-10 (or higher) can cause problems like:
Stalling, prematurely worn engine parts, rusting, clogging of fuel filters and carburetor jets, release of gunk and sludge throughout the engine, frequent water-contamination/phase separation of fuel, and eventually engine breakdowns and even some deaths.
Ethanol can cause a motor to run lean on fuel, due to water, will not burn, which will take the place of fuel, as marine fuel tank pick-ups are mostly at the bottom of the fuel tank.
Vapor lock (fuel starvation) is common when using ethanol fuels.
Alcohol fuels are very prone to phase separation, when the weight of the ethanol and water will sink to the bottom of the fuel tank and get picked up by the motors fuel system. (Even small amounts of water can harm the fuel system).
The initial symptoms, (of using a higher than acceptable concentration of alcohol in fuel, is usually engine stalling when you demand acceleration (WOT).
You'll notice other performance issues, such as increased stalling, misfire, hesitation and difficulty maintaining boat speed during trolling.
The long term dangers of ethanol (and other alcohol-blended fuels) are many, including deterioration of parts (rubber, aluminum, fiberglass etc.), rusting, fuel system clogging, and other varied damage to engine parts and components.
Older engines are more prone to ethanol alcohol damage.
The most reported and troublesome issue with marine engines and ethanol fuel has been regarding the decomposition of certain fiberglass fuel tanks. There really is no solution to this issue, other than to replace the tank (very costly, timeconsuming project); Lining or sealing the tank, for added protection, is sometimes possible.
These warnings and "possible" problems are enough to scare most boaters away from E-10 gas and have them looking for gas stations and marinas that sell "pure" gasoline.
If you talk with your local boat-repair shop, you will likely hear the suggestion to stay away from the E-10 blend. Some go even further. The Dare Marina near Yorktown is encouraging it's customers to write to Congress to ban E-10.They say E-10 has caused "serious and expensive issues in the boating community."
The marina shows a picture (to the right) which they say demonstrates one of the many problems that boaters have encountered, in this case the affinity that E10 has for water and the resulting of phase separation that can occur.
One other concern is the fact that boat fuel, unlike fuel in your car, is vented and the speed of the boat causes the ethanol to attract water more quickly in your fuel system.
The video to the left shows one boat repairman demonstrating why he tells folks not to use E-10 in their boats.
In the end, the decision is yours. But if you look, you can find gas stations and marinas that sell "pure" gasoline.
And Evinrude-Johnson is among the most accepting of boat motor makers. For example Pleasure-Craft, Suziki and Nissan do not "reccomend" the use of fuel with any percentage of ethanol. 
These warnings and "possible" problems are enough to scare most boaters away from E-10 gas and have them looking for gas stations and marinas that sell "pure" gasoline.