My Ebo-Jager has a minimum waterline marked on it. Not sure how accurate the mark is, I always have it completely submerged. I don't think I would risk assuming it's not accurate though.
<edit> I just went and took a look at it, and the mark is pretty high up, not far below the adjustment dial. Not sure about it being a matter of just keeping the coils covered, probably more an issue of where the thermostat is located.