The international ocean community gathered in Nice, France, in June 2025 under a One Ocean framing that unified participants around work for sustainable and equitable oceans. We explore ways that the One Ocean Science shared at the One Ocean Science Congress (OOSC) – including new data technologies promising ‘complete’ ocean renderings – makes space for difference in ocean knowledges and ways of knowing. Drawing on our work on new data technologies, oceans governance, and our participant observation at OOSC and the Third United Nations Oceans Conference, we explore three sites where universalism and difference were evident at the meetings: the propensity to render the oceans technical rather than political; the persistence of gaps as a defining feature of One Ocean Science; and sciences and knowledges incommensurable with One Ocean Science universalism. We highlight that One Ocean Science universalism is, perhaps counterintuitively, constituted through differences and heterogeneities in scientific practices and politics. Manifesting the United Nations Decade of Ocean Science for Sustainable Development’s goals of ocean science that fosters equity and sustainability will require holding space for difference within One Ocean, including by interrogating and redesigning the epistemological infrastructure of One Ocean Science so that pluralism becomes structural rather than residual.