The border is a device to articulate differences. It is increasingly mobilised as a spatial tool to define structures of territorial power, both on land and at sea. Rather than a linear border, however, difference in the ocean mostly occurs across well-defined three-dimensional volumes. Water masses are spatially defined by temperature and salinity gradients across oscillating margins. Either narrow or broad, these margins are relatively constant, and can be measured and drawn according to oceanographic parameters. This chapter examines ways of thinking through borders and ocean governance in Norskehavet (the Norwegian Sea). It contrasts the layering of anthropogenic bordering practices across the sea’s currents, depths, and water masses, driven by oceanographic principles. Based on a more-than-human perspective, explorations carried out by architecture students propose a loose framework of three zones that closely follow the Norwegian Sea’s inherent spatial organisation. Applying a morphology of oscillating margins would mean to align human activities more closely to the ocean’s inherent spatial categories and temporal rhythms. Within these categories, two speculative projects exemplify more radical forms of counter-mapping that tackle the critical issues of salmon aquaculture and deep-sea mining, thereby submerging the observer and exposing the limitations to inherited conceptualisations of ocean borders.