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.
Adjusting the idle speed on the diesel engine is not difficult. The adjustment is located on the driver side of the injector pump near the rear of the pump. You will need a 10 mm wrench. Correct speed is: 1994-1998 with AT - in drive with AC on 750 -800 RPM. 1994-1998 with manual in neutral with AC on 750-800 RPM.