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.
A pattern maker by trade, Ole Evinrude finished his first outboard motor in 1907 And even to this day, outboard motors are basically the same: using Oles idea of a vertical crankshaft, horizontal flywheels, and set of bevel gears. Today Evinrude motors will propel a boat a bit faster than the 07 version (150 mph versus 5 mph).