We don’t see the sea, not really for what it is, we see it’s surface.  Have you ever had that impossible sense of fall-less vertigo in water? The sudden realisation of the depth beneath. Once snorkelling near an island in Crete I followed a shoal of horse mackerel along the line of the seabed perhaps four or five meters deep when suddenly the shoal dashed through some weeds and following them I swam over the ledge of the plateau and the sea bed fell away as if it, not I, was in motion, to a deep blue black hole and I felt as if I could fall into. Like that moment high in the theatre circle when it seems entirely possible you can just step forward and be tumbling down onto the stage. I doubt the sea was more than twenty meters beneath me, and perhaps it was the sudden change in light and heat that the deeper sea brought, but I was deeply afraid of how strange it was.