<p>Estuaries, the dynamic transitional zones where rivers converge with oceans, represent complex ecosystems characterized by the mixing of fresh and saltwater, resulting in what is known as brackish water. These coastal interfaces, along with tidal flats and other littoral features, embody a unique duality, existing as neither fully terrestrial nor entirely marine environments. This ambiguous nature poses significant challenges for scientific inquiry when coastal regions become the focus of study. Inspired by Stefan Helmreich’s (2011) call to ‘think with seawater’, we propose the concept of “brackish knowledge” as a way to engage with knowledge practices that are entangled with both the material complexity of the environments they describe and the practical contingencies of the contemporary science system. In this paper, we follow the development and maintenance of the General Estuarine Transport Model (GETM), a hydrodynamic model designed to represent the complex tidal processes in estuaries and shallow shelf-sea areas such as those along the Dutch coast. We show how the model’s developers move across and recombine properties that are often framed in opposition to one another, namely the physical and ecological, social and computational, and public and private, in order to continue making knowledge about coastal and estuarine environments. We conclude that the material, epistemic, and institutional dimensions of brackish knowledge should be considered alongside one another in the governing of scientific knowledge about environmental change, since this ultimately shapes what can be known about potential coastal futures.</p>