This dissertation centers the Strait of Gibraltar as a space of “material imagination” (Bachelard 1983) through which to examine the political ecologies and oceanographic science of the region. Rather than depict the Strait as a setting in which politics and economy occur, this dissertation asks how the particular seawater of the Strait and of the Mediterranean informs the Strait’s political and scientific importance. To make sure that the specific seawaters of the Strait and Mediterranean “matter,” I draw on the concepts, histories, and images of oceanographic science. This dissertation is based on a year of multi-sited and remote fieldwork that followed alongside the waters of the Strait, including research from such sites as Gibraltar, Cádiz, and Málaga. Throughout the chapters in this dissertation, I ask not only how the Strait functions as an important site of scientific knowledge about oceans and seas, but also how this knowledge about seas and oceans can form a kind of “seawater thinking” that can be used to think through ocean sensing, Mediterranean migration, the role of oceans and seas in climate change, and the political and environmental effects of living alongside seawater.