I would think it would be difficult to get all the water and sludge out by pumping or siphoning, and thats the stuff the ethanol eats up. I actually just switched my small boat from the built-in tank to portable after having problems this weekend. The tanks 15 years old, and built into the hull so cant really inpsect or clean it.
ah, but it's the old type of fuel, so the water (if any) and fuel would be separated. and if you have enough hose, it is no problem getting 99.x% of the stuff in your tank out of it.