Id second atols thoughts. Similar issues can also be caused by the typical steel punchings that are used for ballast rusting and expanding, but normally this shows up in the bilge
If its the only bad patch it can probably be fixed. If there are other signs of cracks and rust weeping out of the hull in many other places as well things might be getting beyond a reasonable economic fix.
A lot depends on how well the boat was built in the first place, and if things like hollow pipe frames were used(a potential rust issue), and if usefull aditives like cromium trioxide to reduce things like hydrogen bubbling along the metal to cement bond which can result in a creeping rust problem thats hard to stop. Was it single
shot or double shot plastered? Get hold of bruce binghams book on cement boats if you can find an old copy.
Is there any sign of some steel reinforcement left in the crack?
The steel reinforcement is the critical part of a ferro cement yacht. If its ok the boat has a chance. If its all rusting away you're possibly best cutting your losses.