This article pivots around the work of early modern legal scholar Hugo Grotius to consider the political stakes of ontological assessments of the sea and water in the context of Dutch imperialism. It draws on links with land reclamation projects in the Netherlands, while at the same time ties these to urgent questions within contemporary critical water and ocean studies around water, ontology, and race. Suggesting a rethinking of Grotius’s understanding of the ocean as perpetual res nullius – perpetually ownerless property – it destabilizes renditions of Grotius’s free sea as free from ownership. The ocean remains firmly within the orbit of property, the property of mankind, thereby excluding those considered non-human, including racialized, gendered, and more-than-human life forms. Grotius’s mare liberum as perpetual res nullius does not form an exception from territorial, personal, and national conceptualizations of property, but rather preconditions it – preparing the world for its thingification. I examine how this understanding of the ocean and of water has colonized our thinking of the ocean, law, being, and belonging. At the same time, the ocean’s very materiality seems to resist Grotius’s legal narrative. Although Grotius’s conquest of maritime imagination continues to justify global models of capital accumulation, the ocean always already shores up against and spills out of such reductive imaginaries.