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.
What year is my Outboard? To find an exact anwser to your question is important to know the brand of your outboard motor, horse power (h.p.) and ehether is a two stroke or four stroke. Look for a little plate on the motor'' mounting bracket or also at the crankcase stamped (earlier outboard models) here you will find an outboard model year guide: Evinrude Johnson OMC model guide