Despite the vast space the seas cover, more than ever there is competition for maritime space for shipping, renewable energy, fisheries and aquaculture, environmental protection of habitats and species and mining. This research aims to assist in harmonizing the competing interests at sea, specifically with respect to the exclusive economic zone, the high seas and the seabed of areas beyond national jurisdiction. This will be done by clarifying the obligation to have ‘due regard’ for the interests of other actors at sea, a concept employed in the UN Convention on the Law of the Sea (LOSC) which is the sole concept at hand to resolve conflicts of uses at sea – yet which remains undefined by the LOSC. This leaves actors to find an unpredictable ad hoc solution which may not sufficiently take into account the possibility of additional other future uses of the seas. This research aims to provide clarity on how the duty to have due regard for other uses of the ocean can be fulfilled – a concept essential to good marine spatial planning. This will be done by contemplating already existing regional and national practice on multi use of the seas, as well as draw from contextually neighbouring areas of law such as the law on shared resources, and in particular shared watercourses and its relevant case law.