Yes you are correct that people have had problems with ethanol in boat engines, but from what I understand it is a chemical reaction with the seal in the outboards that is the issue. Nothing to do with it sitting up. Supposedly it eats the seals up and causes problems. Why outboards have different seals than a regular internal combustion engine I have no clue. As for mileage, as crazy as it sounds, I just took a 1500 mile trip from Texas to NC with my 36' TropiCal National and I seemed to be about a .5 mpg INCREASE using 10% ethanol fuel. The terrain was all the same and my driving habits did not change. I drove a steady 70 mph, did not get into her hard and did not allow cruise control to kick her out of overdrive when climbing a grade. (Though there were no real grades to speak of, we avoided the mountains and the entire trip was mostly over flat terrain. I averaged around 6mpg using regular unleaded and 6.5mpg using 10% ethanol. We filled up I think around 7 times and used 10% ethanol 3 of the 7 fill-ups. Go figure, but I religiously calculated mileage each time we filled up and reset the trip odometer each time as well. All three times we averaged .5mpg better with the ethanol. I started looking for ehtanol after the 2nd time I calculated my mileage but it became harder and harder to find the closer we got to our home in NC.