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.
It is recommended to use an aftermarket idle control when using a 28mm carb. Idle should be set at approximately. 4000 - 4200 RPM with clutch engagement at approximately 4500 - 5200 RPM. Gearing: For fastest E.T., use a 68 tooth 415 rear sprocket and a 9 tooth front sprocket for a gear ratio of 7.55 or a 90 tooth.