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'm not sure about diesel cars,but diesel trucks idle at about 500-600 rpm. Its perfectly normal for it to run at 1000rpms, thats the norm, it should be pretty quiet check ur muffler if u think it sounds loud, but it will not hurt ur motor or trans.