As far as digging it deeper, I can give you some basic advice. They do have extended reach excavators, so that is an option. They are normally a more expensive route to go. I would look into hiring someone with a dragline. Very effective for what you want to do.
Both methods would require access to the pond by means of a flat area next to the water so they can work. Not all ponds have this, contact some excavator companies to look over the site. You can do this with the pond full of water, but it is always easier to do if it is drained as much as possible. The muck and dirt that is stirred up while doing any of these things will not be good for the fish that are in there.
