You will find a more direct realtionship between speed and fuel consumption
than
LOA and fuel consumption.

In fact, larger boats (like trawlers) more often run in displacement mode than
on plane, while those 16-20 footers running on plane will burn more. There are
many examples of boat over 40 feet than can achieve fuel economies of 4-5 nmpg,
but unless you put up a sail or find some unusual exception, it simply won't
get any better than that.

A larger boat usually opens up the option for a diesel engine, and that will
save you some fuel in most cases. The engine itself is quite a bit more
expensive than a gasoline substitute, so those old homilies you hear about
never being able to save enough fuel to offset the cost of a diesel are
generally true. (You may save in the end because the diesel could run 2-3 times
longer than the gas without replacement or rebuild, and resale will be better.)

For smaller boats, the two most fuel efficient speeds are dead idle, followed
by the point where the boat first gets over the bow wave to achieve plane. It
might be accurate to say that many planing hull boaters use twice the fuel they
need (on a nmpg basis) to achieve planing mode by adding that additional
500-1000 rpm to increase speeds beyond that point.

As far as "if you have to worry about the cost of fuel, you can't afford to
boat", there is some truth to that. Even if you're getting
0.5 nmpg and fuel costs $2.50 a gallon, many boaters would find that expenses
for repairs, moorage, maintenance, and insurance (and possbily a boat payment
as well) will be more significant than the cost of fuel.