Although the Greeks lived by the sea, the deep-sea realm was always difficult for them to penetrate, and they imagined it as an alien world inhabited by godlike figures and hybrid beings. Only very rare mortals had access to it: a few heroes, some humans turned into sea creatures, and people who were divers by trade. They all had extraordinary abilities, particularly as far as their “range of vision” was concerned. Focusing on this means of perception, we discuss several ways of ‘seeing’: from divers’ visual acuity to the divinatory knowledge attributed to sea deities (of whom Glaucus was the main representative), not forgetting the colour ‘glaucous’ specifically used to describe the waves, or someone’s gaze. Highlighting the coherence of these elements, we will discuss how the deep sea inspired distinctive visions: although it was difficult to bring back precise descriptions and clear images in line with the terrestrial view, there was a form of clairvoyance involved.