Sorry, I'm no expert, but isn't mpg a bit meaningless, as distance isn't a true representation of work by the engine.
For example, wind and tide both affects the friction/resistance of boat going where it's going. I hear some of you say 'But it should equals out as getting there and then back like they do in speed runs?!' Yes, and no, wind direction changes as well as tide. You could well be against tide out on way out and way back.
Maybe how much fuel it uses in an hour at full throttle is a better representation?!