The island city-state of Singapore is a futuristic, industrialized, densely populated port city. Water independence is central to security. The national government devotes considerable resources to mobilizing urban surface waters. All major rivers have been integrated into a technologically sophisticated aquatic enclave called the hydrohub. Recreational and educational spaces, aesthetically designed into the system, contribute to public acceptance of this radically altered ecosystem. Drawing on ethnographic fieldwork, I show how flash floods can undermine dominant cultural, political, and technical registers that systematically emphasize hydrohub benefits. The hydrohub frames sustainability as a set of interior problems to be solved (assuring clean water and preventing flooding) while disregarding possible unintended consequences of excluding the exterior (probable decline of biodiversity and fishery resources). Yet, enclave ecologies such as the hydrohub may become an increasingly popular model given worldwide water pollution and sea level rise accompanying climate change. By bringing technical aspects of engineering into cultural analyses, applied anthropologists can more effectively reveal the trade-offs associated with ecological enclaves and contribute to models of sustainability that better integrate the maritime surround into the urban fabric of islands and coasts.