Coastal regions support much of the world’s population, economy and industry, yet their water systems face increasing stress from human activities and climate change. Traditional water research commonly is limited up to the river mouth, overlooking downstream impacts on bays and coastal ecosystems. This project pioneers integrated monitoring and data application across river, estuary and bay systems in five countries. Each member country selects a representative land‑sea continuum as a living laboratory. We define coastal bays as the sink and health indicator of the entire water resources system. Under a unified framework that progresses from monitoring to data-driven modeling, we co-develop standardized protocols for core water quality indicators. Each national team contributes with its unique expertise to a complementary module. All data and knowledge are integrated through a central data platform, enabling cross‑country comparison, joint analysis and exchange of techniques and knowledge. The project delivers novel autonomous sensors, AI‑driven predictive models, new ecological and chemical indicators, a quantitative understanding of human versus climate drivers, an open-access dataset following FAIR principles, a toolkit for land-sea integrated management, and a cross-disciplinary training program. All monitoring data ultimately serve to protect water resources, ensure drinking water security, safeguard marine ecosystems, and enhance science‑based decision support. The results of this project will establish a transferable paradigm for coastal water monitoring in the countries involved in the project and beyond.