Next on the list of factors is pressure. When you increase the surrounding pressure, you can usually dissolve more gases in the liquid. Think about your soda can. It is able to keep the fizz inside, because the contents of the can are under higher pressure.
Assuming a prop, it depends on how you are propped but with a redline of 6000 rpm, most outboards perform best with a cruise in the 4000-4400 rpm range. If you want top fuel economy, get a floscan or such and see what is the most efficient for your engine and prop.
Confidence votes 30. According to The Old Outboard Book, Third Edition by Peter Hunn, S. Emerson Harthan of Worcester, Massachusetts invented the first steam powered outboard motor back in 1887. He received a patent for his design, but never actually built the motor, though it did work on paper.