I have seen people asking here about plants which goldfish will not eat or destroy. My big goldfish will try to eat anything and push the plants around all the time, and the plants which have survived unharmed are Water Chestnut (floating) and Moss Ball. They do not even eat the roots of the Water Chestnut. They will eat the algae off the moss ball without damaging it, like vacuum-cleaning a carpet.
If you have water chestnut, keep it under control because too many floating plants can prevent light and oxygen getting to the fish and submerged plants.
Hope this is useful for those looking for plants for goldfish. I will post any more I discover.