A basic rule of thumb is: if you can see the deck of the ship, it can see you too. You're not so far as you though. I think that you're only trully impossible to see, if all you can see from the other target is its mast, the rest being hidden behind the earth's curvature - as are you.
Hum. ...
Only now I thought of this: does SH4 has a curved map, or a flat, medieval map?