Esshup,
So when your pond is low it is still fairly deep? It would not look so drastic without the island even though it is cool.
So there really is not an easy fix for your ups and downs? Huge volumn of clay or giant liner both very costly?
I too have a pond that comes up against the change in ground water level. I am at a loss but it is interesting at least. I guess.
Good luck. Even low I think your pond looks good.
jim