Each boat is a bit different, but he is right. If there is nobody at the helm, that engine should be off. If you want it to sit still with the engine on, you will play a little game with the F-N-R lever to keep it still. It can be done, but someone needs to be working it constantly to make it happen.
Good luck with the practice. Find an object in open water and work your boat around it. Throw a big beach ball out and try maneuvering around it at idle speed. It will be great practice for you when you go into dock. Most have found the ability to pull safe 180's and side moves in tight spaces once you know the limitations of the boat.
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