While this doesn't directly answer your question it may be food
There are two separate issues that need to be addressed with the goal being to optimize fuel consumption, engine efficiency (shaft output horsepower/fuel flow rate) and boat speed versus the amount of push required.
Consider a graph that shows fuel consumption versus boat speed. To go faster requires more horsepower and the fuel consumption gal/hr increases. When hull speed
is approached this curve becomes steep...lots more power is required for a small increase in speed.
However, the faster you go the less time it takes to go from point A to point B.
So one could make a second plot that shows fuel consumed to go a fixed distance at different speeds. This second plot combines the changes in the efficiency of the engine running at different RPM and output horsepower with horsepower required to drive the boat through the water
at each speed.
With the propeller
matched to the transmission
and boat there may exist an optimal throttle setting which gives the best fuel consumption for distance traveled at a reasonable cruising speed. The reason for this is the way an engine functions from a thermodynamic perspective.
Your engine manufacturer has generated a set of curves called the brake specific fuel consumption, BSFC. It is a set of closed curves plotted on RPM versus crankshaft output. For example, you can run an engine at a fixed RPM and output a range of shaft power. Anyway, this chart shows where the engine sweet spot is or the best efficiency, the highest HP/fuel flow rate ratio.
Combining the BSFC information with your power versus speed for the boat will allow you to figure out how efficient you are. However, power versus speed is difficult to obtain unless you instrument the shaft.
This brings us to your fuel flow meter. My guess is it is showing fuel consumption for an estimated curve through the BSFC plot or more simply for a constant engine efficiency and it really doesn't have much to do with the reality of the boat performance.
I would do a series of experiments going from point A to point B on calm water
at 5 or 6 rpm settings and record
the time to get from point A to point B (or read the speed from the GPS
in no current
water and calculate the time to go 1 mile). Record
your fuel flow meter reading at each speed. Create a plot showing fuel consumed for one mile versus speed or time to go 1 mile and pick the best speed.
To estimate fuel consumed (incorrect by a constant factor but doesn't matter) multiply your fuel flow reading by the time it takes to go one mile at each rpm setting.