According to the European Environment Agency, the seas have become busier places, driven by a combination of technological advances and society’s increasing demand for food, energy and other resources. The EU-funded AQUATIC project will investigate the environmental threats to marine ecosystems in the Baltic and Mediterranean Seas. It seeks to understand the effect of social categorisation on socio-material practices. Specifically, it will focus on two cases – both under threat but marked by opposite rationales regarding the formulation of the threat: eutrophication in the Baltic Sea as a case where life growth is described as harmful, and the threat posed to Posidonia oceanica in the Mediterranean as a case where life needs protection.