20 seacraft with a suzuki 140 or 150 etec will get 5mpg at cruise.
I don't know if it'll fit in a 20 seacraft, but the cummins mercruiser has a nice little 130/150hp package
http://www.cmdmarine.com/qsd2.0sterndrive.html
Probably better economy . . . but that is just a guess.
Hey . . . Do you email me about a 20 Master Angler in Sacramento?