This article explores the intra-active collective politics of the Loweswater Care Project (LCP), a ‘new collective’ of humans and nonhumans that assembled in the English Lake District in 2007 to grapple with the potentially toxic blue-green algae (cyanobacteria) that were proliferating in Loweswater lake. The LCP was motivated by questions similar to those asked by the editors of this Special Issue on ‘Water Matters’ in their call for contributions and to which this article responds, namely ‘how can we acknowledge the agency of more-than-humans in our political ecologies?’ and ‘how can this help us compose better, more balanced, human–environment interactions?’ To answer these questions, the paper examines how the LCP put intra-active collective politics into practice, a form of ontological politics informed by the work of Bruno Latour on object-orientated politics and Karen Barad on agential realism. It explains the key role played in ontological politics by what Barad calls agential cuts and what Law refers to as a method assemblage, both of which can be used grasp the intra-acting agencies entangled in matters of concern. Two examples are given to illustrate this: first, the scientific modelling the LCP undertook to understand connections between land use and water quality, and second, the hydro-geomorphology survey it conducted of the catchment to grasp the links between hydrological processes, land forms, and earth materials. While the first example highlights how method assemblages perform agential cuts and craft realities and presences for new collectives to do ontological politics with, the second illustrates how the realities crafted by the hydro-geomorphology survey impacted on the LCP's sense of collective agency. The paper ends by reflecting on the ethical dimensions of intra-active collective politics directed at composing a better common world and on the issue of ‘care’, both of which require further attention.