Ron Thornton says:

It would seem to me that GPH is not a very useful measurement but one
boaters have been stuck with because of the fluid nature of their
roadway. Now comes along very accurate distance measurement via GPS and
you should be able to come up with GPM just like a car. This to me
seems like a much more useful piece of info. Yowweee, another gismo
for the dash in the boat.

Now, that may work on a lake, but if you factor in the tide, you could have to
travel _miles_ further to get home than you did to get out there. It works
for the gph thing as well.....
Personally, I would just as soon have a reliable gas guage in the dashboard.
And know how to navigate before you buy the boat......

Steve ;-)