Shared water has prompted some neighboring countries to form integrated regional institutions and rules. Other neighboring countries have maintained less cooperative policies in managing shared water. When do governments achieve integrated management of regional water? How do they overcome barriers to integrated cooperation? This article contends that the structure of interdependence, not the number of states bordering the water, encourages different international policies. It analyzes original panel data on the agreement histories of seventy-six rivers, lakes, and seas. It also examines the role of selective incentives in promoting mutual water management. Fundamentally, the number of states with access to the water does not determine the fate of water management. Strategies for overcoming divergent preferences can be applied across situations. The key is to provide selective incentives when states are asymmetrically interdependent in using the water, to alter national preferences. Selective incentives are particularly important when few states share the water.