OTTERS will aim at promoting and scaling up successful citizen science (CS) initiatives in the marine and freshwater domains via 1) accelerating the co-creation of standards in data collection, semantics, data quality, and data management making sure to abide by all ethical and legal standards; 2) promoting and scaling up successful water-related CS initiatives by clustering them under co-designed Spring-to-Sea campaigns to foster agency and increase ocean literacy; 3) connecting the citizen-generated data to other EU-funded projects and portals, including the European Marine Observation and Data Network (EMODnet) and Digital Twins of the Ocean (ILIAD) to ensure the accessibility and reuse of the data and abide by all FAIR principles; 4) demonstrating the effectiveness of citizen science in participatory research, environmental monitoring and changing people’s behaviors to ensure the sustainability of water ecosystems and reach the goals of the EU Water Framework Directive and UN Decade of the Ocean.