In general, studies on regional integration and transboundary water management (TWM) focus on the relationships between large states. Instead, this paper analyses TWM in relation to two cross-border cases involving small states: Germany–Luxembourg and Mexico–Belize. Small states are significant due to their presumed adherence to regional governance and vulnerability to external shocks, especially those of climate change and environmental threats. Specifically, the article asks: How well do small states implement regional water governance models, especially in cross-border contexts? It concludes that the interpersonal bargaining and consensus-building that characterizes small state governance limits effective TWM.