It is widely acknowledged that marine spatial planning (MSP) should be responsive to the dynamism of the marine environment and the relatively tenuous human relationship with the sea. However, MSP remains conceptualised within rationalistic terms that limit this potential. This article places MSP within the context of spatial theory that holds greater promise for developing more progressive practice. Firstly, the interrelation of MSP with current notions of soft space (less formal, cross-cutting spatialities) is explored, suggesting that MSP is expressing some of the geographical and institutional freedoms of soft space, and may contribute new insights to this concept. Secondly, a progressive framework is developed that builds upon soft space principles. This draws in, on the one hand, underlying relational understandings of space, and on the other hand, insights from marine contexts. This leads to a picture of marine space-being-planned as lively space, expressing, amongst other things, the sea’s materiality. This concept is illustrated through an application of the framework to a strategic MSP exercise for the Baltic Sea region. Finally, it is suggested that MSP itself should be reconceptualised as immersed into this spatial ontology, with the agents and practices of planning taking their place within the wider assemblage of marine actants and relations.