The oceans are littered with ultra sharp spikes. Their refraction index is the same as the water so this makes them invisible when hovering in the water. These spikes shred through flesh or steel. They impale themselves into whatever material comes into contact with them. Because of this sailing or swimming in the seas is almost unheard of. Almost anyone who treks into the sea is guranteed to sink, and suffer in a lot of pain while it happens.
This was caused by the divine magic of a god during the great war.The god is long dead but their deeds continue to wreck havoc on the living. These spikes have infact caused a forced migration of many intelligent species living in the seas, many died who were able to migrate.