This article demonstrates the empirical relevance and elaborates the theoretical foundation of a “polity-centered” causal mechanism of international environmental regulation which has been only superficially touched upon in international environmental regime theory and which challenges the policy-centrism of this field of research. Motorboat regulations on Lake Constance demonstrate the limits of established approaches in regime theory in explaining the strict regulations of this early regime. Rationalist explanatory approaches are not convincing since there are no helpful structural constellations and no functional need. According to normative-cognitive approaches, the institutional density and differentiation that exists in the transboundary Lake Constance region makes an “advocacy coalition” approach better suited than an “epistemic community” approach. Yet, even this perspective cannot explain the international breakthroughs towards strong regulations. To fill the remaining gap, it is necessary to account for the symbolic value of water in representing emerging transnational identities and institutions. Polity-centered coalitions of political leaders around the lake “performed” innovative regulations in a highly symbolic policy field in order to gain attention and recognition for their institutionalization of the idea of a “Euregio Bodensee.” The article ends by demonstrating the empirical relevance of this causal mechanism beyond Lake Constance and discusses the theoretical consequences in the field of transnational water governance.