AbstractRecent scholarship in law and society has engaged in novel ways with maritime spaces, articulating how they inform legal theory more broadly. This essay builds on such scholarship, and on a broad-brushed survey of maritime history, to make two basic arguments. First, a look at political and legal processes regarding maritime spaces reveals that law is transnational ‘all the way down’. Legal theorists often assume that transnational legal processes are an added layer beyond domestic and international law. But the maritime perspective reveals that transnationalism comes first, both analytically and historically, as a constant negotiation of the relationship between what is ‘inside’ and what is ‘outside’ a polity. Second, the maritime space begins, at least in dominant legal traditions, as an absolute exteriority – imagined as outside or beyond polities and jurisdictions. But with the climate crisis and the emergence of the Anthropocene we may observe an inversion, the sea now appears as a record of harmful human activity; a mirror showing a troublesome collective portrait of humanity. The inversion from a maritime exteriority to the intimacy of ubiquitous environmental harm defines the parameters of law and politics today. The essay concludes with reflections on how the maritime perspective may best be engaged today in responding to that image through political action. It conceptualizes what I call the ‘commonist lifeboat’ – a model of bottom-up universalism for tumultuous times.