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.
The outboard motor is the most popular and widely used engine for propelling small boats. It is designed to be attached outside the boat transom and consists of three components: a self-contained unit that holds the engine itself, a propeller and gear box. Outboard motors are small, light in weight and quiet.