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.
I looked at that motor and thought to myself it is probably toast. Over the years, my rule of thumb has been that an outboard more than ten years old or one that has more than 1000 hours on it is probably worn out and will need to be replaced, or at least rebuilt, in the near future. I know there are all kinds of exceptions to this like lhg's merc's with over 1200 hours on them.