Delhi fishing ships stop being able to fish - possibly after researching one of the dock techs. I will try to replicate if I have time.
I was able to replicate bug where Delhi fishing boats stop automatically fishing. Initially, I thought it was because of selecting all military units (which needs to be fixed as well), but this was definitely not the case. I would have to re-task individual fishing boats to restart fishing, and this would sometimes work and sometimes not. The fishing boats just refused to continue fishing despite plenty of fish present.
Do you know after which Tech they stop?
I haven’t had time to test this out systematically yet, but I’ll try to remember next time I play Delhi on a water map.
I also noticed that stone walls near the water can also cause pathing issues for the fishing boats in general.