so if its 100' diameter and I have a 550' pond, would that mean that I'd need one every 200'? Which would be 3?

What do these things cost?

Whats the surface look like? Is it a bunch of bubbles coming up out of the water everywhere?

Is there an ideal depth that the pond must be?

One thing I wanted to do was leave one end of the pond around 3' - 4' deep for young kids to swim in. Will this shallow area just attract unwanted vegetation?