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.
Outboard and Inboard Engines; Stern and Jet Drives. 1  An outboard is a portable, self-contained package of an engine, gear case, and propeller that is attached to the transom of a boat.  A growing number of outboard engines are of four-stroke design, but many are still conventional two-stroke engines that burn oil as a lubricant along with the fuel.