Marine spatial planning (MSP) in Europe is in a paradigm shift as all (coastal) European countries now have established practices for the production of marine spatial plans. Though international guidelines and an EU directive for MSP provides policy frameworks, the formulation of national policy designs for MSP remains a national responsibility resulting in vastly different practices. Focusing on three Northern European countries; Denmark, Germany and Norway, this paper presents examples of how national policy designs for marine spatial planning are structured, and how the current practice in each country is influenced by local planning cultures. This mapping gives insights to a number of challenges facing planning authorities when planning for sustainable development. Ambiguity dominates the framework of marine spatial planning and the central sustainability concepts it contains. This paper gives voice to the planning teams, as they are key-players in generating meaning in this ocean of ambiguity, giving insights to their understanding of sustainability in the planning of futures for sustainable seas.