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.
An outboard motor is a single unit consisting of engine, shaft and propeller (and sometimes fuel tank) mounted on a pivot on the stern of a boat. To steer, the entire motor is pivoted using either a tiller fixed directly to the motor, or for larger units, a hydraulic or rod coupling from a wheel.