"Any other airborne craft, including fixed wing, gliders, hang-gliders, hot air balloons and airships, must not fly lower than 300 metres or within a 300 metre radius of any Dolphin or Whale. All aircraft should avoid approaching any Whale or Dolphin from head on and should, wherever possible, prevent the shadow of the aircraft from passing directly over the animal."
So 900' is the law. 1500' is a happier height for the glide back to the beach when the ponies escape and the front goes quiet.