In occupied territories, the oceans—and particularly marine species—face significant but often overlooked threats, including environmental pollution, ocean acidification, and the overexploitation of fish stocks. The Law of Occupation is often described as a tripartite balance, where the interests of the Occupying Power, the ousted government, and the protected population constantly meet. Nevertheless, there is no mention of any obligation that explicitly focusses on the oceans or marine species. As a result, the existing legal framework appears insufficient to face the threats that sea animals experience during occupations. This chapter aims to reflect on the protection of marine life in occupied territories and proposes practical solutions within the existing legal framework. Specifically, it examines how the Occupying Power’s legal role under Articles 43 and 55 of the Hague Regulations can be adapted to address the mentioned challenges by interacting with other areas of international law, specifically the Law of the Sea (LoS). The analysis focusses on the due diligence obligations set out in Articles 192 and 194 of the Convention on the Law of the Sea (UNCLOS), which aim to prevent environmental harm. Moreover, the chapter considers how to incorporate these obligations into the Law of Occupation, either through the principle of systemic integration under Article 31(3)(c) of the Vienna Convention on the Law of Treaties (VCLT) or by directly applying UNCLOS to occupied territories.