Sorry, you are correct I used the wrong technical terms. Good catch. I was trying to say, amount of additional energy to overcome (not exert) the forces associated with pulling the boat.
Another way to explain this can be this: the wasted energy, although it will increase with increasing load, increases in a smaller proportion than the used energy.
Let's see where 100 units of energy go
Low load case: 50 units go to heat and 50 units go to motion, moved 10 miles,
overall energy use 100 units per 10 miles = 10 energy/mile
TRAVEL energy use 50 units per 10 miles = 5 energy/mile
Heavy load case: 40 units go to heat and 60 units go to motion, moved 6 miles ,
overall energy use 100 units per 6 miles = 16.7 energy/mile or 67% more energy consumption.
TRAVEL energy use 60 units per 6 miles = 10 energy/mile or 100% more energy consumption.