the boat already has a speedometer? What sort of signal is it? That might be the simplest solution.
While of course you *could* compare indicated to made-good, nobody is asking for it
and you would have to get programming to make sense out of it, for an individual user.
I think if there is a "digital" speed signal on the boat, see if we can use that, else gps up a speed signal is the pragmatic approach. gps would give a reading more in-tune with what us lubbers would consider mpg.