A friend of mine has put a lot of big rocks right around the outside of her pond, she says that Herons don't like to be on uneven ground, this isn't a guarantee though it seems to have worked for many years. Some people also put a rubber snake or 2 near the pond they don't like snakes, or even a model of a full sized Heron as they are territorial.
Good luck.
I wish I had these tips a while ago as I lost 10 Koi carp in a very short space of time, I gave up in the end and filled the pond in, hope you have better luck.
2 things worked for us- a massive metal heron standing in the pond (looks quite nice actually) & also a small electric fence around the edge (daredevil me tested it out & it does not hurt that much)- i think they cant land on water
i'd forget the faux herons, that is a myth in fact they will be drawn to your pond because they see another there!! Right they hate un-even ground, so bolders or varying rocks around the pool to stop them 'wading' in is the best, also to attach stakes/screw/loops around outside of the pond and then criss cross across the pond with cat-gut, you won't see it, but they will and then that blocks their wading route into the water. good luck.