“Since water is a fluid medium the formula of Miles Per Gallon does not work.”
Water can change state so NOT always fluid
“Fuel consumption in boats is measured in Gallons per Hour (GPH).”
A lot of countries use the metric system so litres/km and/or use Imperial gallons which are NOT USA gallons
“It all relates to the size of your engine and nothing else.”
Fuel consumption ALSO depends on other factors such as throttle setting, combustion efficiency etc.
“A single cylinder will burn approximately 1/4 GPH, a 2 cylinder approximately 1/2 GPH and a 3 cylinder approximately 3/4 GPH”
Not true if the cylinder sizes are different.