The 'fishermen will cast their nets there' image refers to fishing over the submerged ruins of the city — the city was thrown into the water, so nets were cast on top of it.

The phrase is not metaphorical for desolation in the sense of an empty beach. The stones and timber of the city were literally cast into the sea to build Alexander's causeway. Fishermen in antiquity cast nets into the water adjacent to/above submerged structures where fish congregate. So the image of fishermen casting nets 'where the city was' is a literal consequence of the city being cast into the ocean — they fished on top of the sunken material.