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.
One word that you want to remember when it comes to boat props is the term pitch. In general, for every 2 of boat propeller pitch, the revolutions per minute will be about 400 RPMs. If we plan to have the best functioning boat engine in the bay, we must decide upon a lower pitch prop.