OK, I assumed everyone here knows that you can hear a prop from a good / safe distance away. When snorkeling in the keys, it seemed like I could hear one boat better than others, and it turned out to be a pod drive boat (two pods), I think IPS.
Whenever they were maneuvering and switching from forward to reverse, the props sounded like it was clattering underwater. An outboard boat was much higher pitch with almost no engine noise, and inboard more bass engine noise and slower prop noise.
I agree that swimming near a pod drive (or any other prop) is a bad idea.